Warning: This Content May Contain Plot Spoliers. Please be aware of this before continuing.
Final Fantasy IV, like many RPGs has a number of "secret" or very rare items. It also has it's share of rumored items that don't exist. Here we'll lay them all out for you.
Thanks to Deathlike for contributing details to this data.
This sword exists in only the Final Fantasy IV Advance version. This is the ultimate holy sword for paladin Cecil. You are awarded it for successfully completing all the tests in Cecil's Lunar Ruins scenario. The sword bumps Cecil's AP to 255, is holy elemental, increases stats, and randomly casts Holy.
This sword exists in only the Final Fantasy IV Advance version (Note: in Japanese it's name is "Excalipoor"). This is one of the awards you will get in Cecil's Lunar Ruins scenario if you don't pass all the tests. The name is a play on words in both languages. It's not exactly what you want to earn. Make sure you pass all the tests correctly and get the Lightbringer.
To obtain this sword you must fulfill the Rat-Tail Collector side quest. Essentially it is the Legend Sword as reforged with Adamantium by the renown Dwarf Blacksmith Kokkol. Learn about forging the Excalibur Sword HERE.
The Defender Sword was the most powerful non-holy sword in the orginal versions of Final Fantasy IV. It is found by diligently searching all the treasures in the Cave of Summons of the Underground. Its chest is guarded by monsters, a formation of five Warriors. However for an AP of 109 it's worth the trouble to get. Not well known is that if you use it as an item while equipped in battle it will cast the white magic Protect spell upon the user.
This odd sword is found by diligently searching out all the treasures of the Sylph's Cave in the Underground. The chest containing it is guarded by strong monsters so be careful. Once obtained you'll find it to be the oddest weapon. Although it can be wielded by both Cecil and Cain, it requires two hands so whomever uses it must forgo the use of a shield. It's not terribly powerful but does have a healthy AP of 80. Finally it makes its user go berserk in battle. It never casts the spell, whomever has it equipped will just automatically attack uncontrolled throughout a battle. This has some advantages, it increases the user's damage output by 50% and helps speed things up while randomly leveling up. It can be problematic when fighting bosses or tougher foes, since the user cannot be controlled, nor snapped out of berserk status while equipped. Though it's more powerful than the Medusa Sword, it's quirky nature make it a weighted choice as to when to use it.
The Sleep Blade sword is found in the ruins of the Eblana Castle,. With an AP of 55 it is not terribly powerful but it has it's uses. Especially because it is often overlooked when it can be the most useful. Eblana Castle can be plundered as soon as you have the Enterprise at your command. If found before the Tower of Zot, it is a slight upgrade to the Mythril Sword (AP: 50) which can be bought in Mythril Village. The Sleep Blade, as it's name implies, has the "Sleep" attribute set and can cause an enemy to fall asleep on hit. When used as an item in battle while equipped it will cast the Sleep spell. Spell effectiveness is driven by level.
Found in a hidden room in the caves of Eblan, this often missed sword is enchanted with the "Drain" spell. This allows it to reap an enemy's HP, based on the strength of the hit, and bestows it upon the wielder. While it's AP is quite low at 45 and it has a terrible accuracy rate (meaning it misses often), the advantage of healing Cecil each time he effectively strikes is appealing. It is also very effective against the flan/pudding monsters, one of the few weapons that is. Be wary, using this item on undead monsters (even the undead flan in FF4 Advance) will reverse its effect and drain Cecil of his HP!
Medusa Sword / Gorgon's Blade / Break Blade
This sword is the extremely rare drop by Medusas, Gorgons, and Black Lizards in random battles (if the creature is known to drop Medusa's Arrows chances are they can RARELY drop this sword). With an AP of 77 it is slightly more powerful than the Ice Brand sword and second only to the Defender Sword in terms of raw attack power for a non-holy normal sword. Depending upon when you are able to win it, this sword may be a windfall power-up for Cecil's attacks. The earliest would be when you get the Enterprise and can land on the Eblana continent. Here you can engage in random battles where Black Lizards are present. In addition to it's high AP, it also has the "Stone" attribute set and randomly causes partial petrification on-hit. Often this effect, if it hits, can destroy some enemies over time.
This weapon is found in a secret chest by walking through a hidden passage in the save room of the Baron Waterway. It is the first non-dark and non-holy sword in the game you can find, and is the first sword you can find after Cecil becomes a paladin. However it has a full 5 points lower attack power than Cecil's legend sword. So what is its purpose? It has the odd "Curse" attribute, and will randomly curse enemies on-hit. This reduces an effected enemy's attack and defense rating by half. Apparently the status effect is supposed to outweigh the relatively low attack power.
Out of all of these items this has to be the most obscure to everyone except probably ROM hackers of the original SFC and SNES ROMs. This is item #70 in the item list (between Lamia Harp and Poison Axe) and is referenced in the ROM by the value 0x40. It normally does not have an item icon and is called "Spare". If you force-equip it to a character it's weapon graphic is the same as Ragnarok Sword and it's attack and sound effect are also the same as Ragnarok. It has the same "Holy" attribute and the same attack power of 200 as Ragnarok. The only difference is it set such that it cannot be equiped. If you modify it's record to allow it to be equipped, you'll find it is a 2-Handed weapon. Essentially a 2-handed Ragnarok that the game never uses! Perhaps this was meant to be used in a demo or perhaps they decided late to make Ragnarok a single-handed sword. No one is sure why it is there. However if you are a ROM hacker it is a sword you can enable and use someplace if you want...which is I guess all it's really good for! Be aware this item is by no means obtainable in any version of the game, and it's existence in up-ported versions is unknown.
Yes this weapon does exist! But ONLY in one version of the game - Final Fantasy IV Easytype on the Super Famicom. In that game the Ancient Sword is re-configured as the "Coral Sword". It has an attack power rating of one point higher than the Legend Sword (since that's the sword Cecil should currently have when you find it). Then it was set to have a "Lightning" attribute instead of "Curse". Since this sword is found in the Baron Waterway as you're infiltrating Baron Castle it is extremely effective against the enemies in that area as well as in the upcoming battle with Caignazzio. Apparently this was done to make the game easier.
Yes this weapon also exists! Again, ONLY in one version of the game - Final Fantasy IV Easytype on the Super Famicom. In that game the Silver (or Mithirl) Sword is re-configured as the "Piglet Sword". It was set to randomly cause "Piggy" status on-hit. The game no longer recognizes it as metal either. Convienent, since you'll probably find it before going to Dark Elf's magnetically-shielded cave where metal weapons can't be used. That would be in a pot in the passageway to Odin's throne below the east tower in Baron castle. Again this change made the game easier since Cecil could wield it in the aforementioned cave.
Only found in Final Fantasy IV Advance, the Piggy's Stick is an homage to the Piglet Sword of the EasyType game and functions the same. It is found in the Chocobo's Forest in the Lunar Ruins.
I have heard of a rumor about an "Imp Sword" or "Goblin Sword" that can be won by fighting some extremely rare Imp / Goblin formation in a random battle. This is completely false. There is no sword in any version of Final Fantasy IV that would fit this description. The only plausible explanation is that someone won the extremely rare Goblin Summon item (to used to teach the Goblin Punch summon to Rydia) from a random battle with Imps / Goblins and didn't know what it was for.
Mage Masher / Mute Knife / Rune Dagger
Found among the treasures of the Sylvan Cave, this dagger is often missed. It is arguably the best dagger in the game and as its name implies, is quite useful against enemies who cast spells. It can inflict "Silence" status on-hit. Interestingly, it's not a bad choice for mature Rydia, particularly in Final Fantasy IV Advance. Another good choice is having Edge equip it in one hand, as the effect against Mages and his better attack power make it again a good choice as well. It is useful when you run across the troublesome Warlock enemy, as she likes to cast Tornado when left alone. Often the weapon will silence their spells making them easy to kill.
This dagger is able to be bought in Mist Village, but the earliest this is available is after you return to Baron with Cecil as a paladin. However the weapon is worth the trouble of trekking to Mist Village (again). It has a relatively high AP of 35 and is a much better "Attack" option for a black mage or sage than a rod or staff (or bow and arrows for that matter). Besides it's utility as a blade weapon it has one really neat trick, the "Dagger Dance" magic spell. When used as an item while equipped it will cast this spell and a single selected enemy is dealt a non-elemental magic attack for around 150-400 points of damage.
Like the "Hand Axe" this weapon seems to exist but is never available in the game. At least it exists in the original SFC/SNES versions of Final Fantasy IV. I've never found it in the game during normal play, it's not sold anywhere, and to my knowledge no monsters drop it. It appears to have an AP of 34, 1 less than the Mage Masher. However it has the "Death" attribute set, and will randomly cause instant death to an enemy.
Found only in Final Fantasy IV Advance, this is Cain's ultimate weapon earned by successfully completing his Lunar Ruins scenario. The spear is lightning elemental and randomly casts the Weak/Tornado spell, which rely upon Cain's and the target's level. The Tornado spell ignores the normal boss check.
This rare lance is randomly dropped by some monsters in the Lunar Subterrane. It is effective against dragons. However it's AP is only 99 making it inferior to the Holy Lance in terms of raw attack power. However it is worth using vs. Dragons.
This is the ultimate Axe in Final Fantasy IV, and is rarely dropped by the Giant Mechanical type monsters. It is extremely effective against mages. It also has a good normal attack power of 100. Unfortunately it is a two handed weapon, making Cecil or Cain go sans-Shield while using it. It's actually a very good choice for Cid in Final Fantasy IV Advance, although there are better hammers he can get. Unfortunately in versions of the game prior to FF4 Advance, it can't be gotten while he's in your party.
This weapon exists in at least all of the original versions of Final Fantasy IV (SFC Normal, SFC Easy, SNES FF-II). I've never found it in the game during normal play, it's not sold anywhere, and to my knowledge no monsters drop it. It does exist. It is the item #56 in the item list and is referenced by byte 0x38 in the ROM. It is a single handed weapon, the only axe-type that is, hence the name I assume. Paladin Cecil, Cain, and Cid can equip it normally. It is also the only axe that Edge can throw using the toss command. It's existence in up-ported game versions is unknown.
Rod of Change
This is the best rod that can be found in the Overworld of the Blue Planet, with an AP of 15. However it is often missed because of its obscure location. It is found in Rydia's House in Mist, which you can return to later in the game after Mist is burned. Rydia's house is empty and the fireplace has gone out. If you walk into the hearth you'll find a secret passage that leads to a few treasure chests. And yet another hidden passage from those chests leads to a single chest containing the Rod of Change. This rod has the "Piggy" attribute set and will randomly cause pig status to an enemy on-hit. Also if used as an item while equipped it will cast the black magic spell "Piggy". The ideal time to get this weapon is when you return to Baron from Mysidia with Cecil as a paladin. Leave Baron and go to Mist Village via the Mist Cave. Then you'll have it to use for either Tellah or Palom.
This rod is the ultra-rare drop by Lilith and Lamia monsters. It is often missed because of this. It has an AP of 13, which isn't bad for a rod. It's attribute is probably more valuable. When used as an Item while equipped in battle it casts the black magic spell "Osmose". Now the only time it would be a practical weapon is if you can manage to win one at the higher levels of Mt. Ordeals where run-ins with Liliths are common. This is because it would be a slight upgrade to the elemental rods available in Mysidia's weapon shop, and might be particularly useful for Tellah with his MP frozen at 90 and for him the spell multiplier of the rod is greater than his natural level. However after Cecil becomes a paladin, Tellah knows the Osmose spell so the only real advantage may be for Palom to use it, although using Osmose via the rod has no casting time. In terms of raw attack power it is replaced by getting the Rod of Change found in Rydia's house in Mist after you travel to Baron from Mysidia.
Dragon Whip / Wyvern Whip / Dragon's Whisker
Samurai / Yoichi's Bow