Biggest Inspirations for this were the Amazon Pairs by Cavegirl and Crocodile Class by Goatman's Goblet
Sometimes the crocodiles who float the River Styx perish. When this happens, their souls split, reincarnating as two individual mortals. Coming together most commonly as twins or lovers, the split souls seek adventure and power before finally returning to their guardian forms. Together, they are known as Styxian Pairs.
Both classes share the following abilities:
Natural Swimmer: Can swim and make attacks underwater without penalty. Can hide in water and swamps on 3-in-6
Bond of the Soul: The Jaws of Death and Quietus Oar cannot recover hit points without the presence of the other. While they maintain their own HP pools, certain Soul-Effecting magics have the chance to harm both equally, forcing each individual to save against the effect even if only one would be a legal target.
True Form: Guardian of the Styx: At 9th Level, the Jaws of Death and Quietus Oar can physically combine to live as they did in a past life: as a massive, 40 foot long crocodile beast. They can also separate back into their humanoid forms, but can only either split or fuse once each day.
Jaws of Death
XP- Magic User
Hit Dice: D6
To Hit- As Thief
Saves- As Thief
Spellcasting- As Cleric, (but see below)
Fangs of the Beast: Can Dual-Wield bladed weapons without penalty. When successfully making two attacks against a single target in this way, the Jaws of Death can immediately attempt to grapple the target for free.
Speaker of Ruin: Jaws of Death have no access to any spells innately and must learn them from other's spellbooks or similar sources. Jaws of Death can learn magic but must scrimshaw spells on bones or teeth. This usually means they are limited to access wizard spells due to the nature of spellbooks and scrolls, but if they find a scroll or learnable spell from a Cleric's spell list they can learn it as well (assuming whatever outside force grants the spell responds to the Jaws of Death's will). They can cast spells underwater without issue.
XP- Magic User
Hit Dice: D8
To Hit- As Fighter
Saves - As Fighter
Mighty Tail: Quietus Oars begin play with a special club that deals d12 damage. It deals max damage on grappled targets
Rush: Quietus Oars can move at twice their normal speed in a round a number of times per day equal to their level.
Navigate the River: Quietus Oars can navigate unerringly in water, swamp, or similar terrains
Design Notes: This is meant for something like B/X or Basic Fantasy and seems a bit more powerful than the standard classes. Having the bond be squishy (two adventurers are twice as likely to die as one, right?) might make up for it or maybe you should gate these classes behind stats or something.
I haven't really written any classes like this before but it was sorta fun. Probably don't expect it to be balanced.
Natural Swimmers' "Hiding" ability was originally just "hide as thief in swamp settings" but a half-chance seemed more straightforward- it should probably be an emergency thing rather than a reliable one. If characters take the time to hide smart then they usually aren't found but this would let the Styxian Pair supernaturally disappear, even if they would normally be in plain sight. Basically an elven cloak ability.
Some of the balance probably depends on how reliable or dangerous Grappling is in your system, but trying to game the Mighty Tail's ability seems like a fun tactical hurdle to me. It might still be a bit too strong.
Since I like progression in classes, I'd probably use Wizzargh's Fighter Techniques with the Quietus Oar and let them access Fighter Techniques at either half or third of the base Fighter rates. Collecting wuxia techniques is just really fun and lines up nicely with Wizards getting new tricks from spells.
The capstone doesn't have specifics but that's on purpose. For some things a "normal" super sized croc is fine, for my system I'd probably just let the players have the normal semi-demigod abilities that styxian crocodiles would have, like eating and controlling ghosts and total immunity to poisons.
I also have some ideas about lore and having individual Jaws of Death from certain regions or backgrounds utilizing different styled weapons- in the first draft they used Shotel's specifically to emphasize a pincer/jaws motif. Maybe there are snake variant's who use whips and poison daggers for their jaws, or a variety of long and short blades to emulate different animal styles. I might follow up on a later post if I come back around to it.