Animocorporeal flux

This feat is not gained in the standard way (through character levelling) and is instead applied independently of the player's wishes by the DM. It is typically gained through exposure to events that would normally heavily affect a character's sanity, such as events that cause indefinite madness as stated in the dungeon master's guide for DnD 5e. It can also be caused by events similar to coming into direct contact with substances or entities from utterly alien realms of existence, or from magical discharges from said realms or other realms of chaos (e.g. the Far Realm or Limbo).

 Having experienced this event, you roll a die and gain the following attributes based on your result, alongside one effect from the short term-madness that repeats every d10 hours, and one long-term madness feature:

1. Gibbering jaws: mouths filled with razor-sharp teeth start to open up on every part of your character's body. Their mouth also takes on the appearance of these,  and these mouths will speak simultaneously with you as you are doing and in your voice, and can be used to perform a bite action where appropriate with the same characteristics as an unarmed strike, but dealing piercing damage instead. You may also attempt to cast Dissonant whispers or Tasha's hideous laughter once per day.
2. Flailing tentacles: D8 large tentacles erupt from your character's back and start to flail around until the character takes a short rest lasting 2 hours learning how to control them. The character's hair, if any, also turns into smaller tentacles that mostly lie flat against their skull unless willed otherwise. All tentacles secrete a mild paralysing liquid that can paralyse a medium-sized entity or under if that entity fails a DC 5 Constitution save against becoming paralysed. The DC for the save increases by 5 for each turn that the tentacles are in contact with the entity. This fluid does not affect your character. The tentacles also grant +1 attacks (unless vetoed by DM), provided that the character has the dual-wielding feat as well. 
3. Glaring eyes: scores of new eyes open up on the character's body seemingly randomly, and are perfectly capable of sight. The character's field of view becomes omnidirectional, and as such, they gain +2 to their passive perception as well as Nightvision for up to 30 feet seeing in darkness as in bright light and for a further 10 feet seeing as if in dim light, but nothing beyond that. The eyes may vary wildly under many aspects (such as colour and shape), or not at all.
4. Shifting: the character appears to constantly spasm into various wild shapes and positions that can only be seen as afterimages, unnerving those around them though seemingly causing no ill effect to the character or their surroundings. They also appear to have a strange after-image in the place of a shadow, which trails after them as they move and lasts for a fraction of a second. These characteristics cause the character to gain +1 to their charisma (intimidation) modifier, and to have a 5% (or 1 on a D20) of avoiding non-area of effect damage, on top of their standard saves.
5. Fibrous physique: the character loses all previous body structures, which are instead replaced with coiling muscular ropes akin to worms. These can then uncoil from the character, lashing out at anything in the surroundings and can be individually manipulated like the flailing tentacles, provided the character takes 2 hours learning how to manipulate them, and confer +1 attacks if the character also has the dual-wielding feat. The ropes can also uncoil further, classing as 30 feet of hempen rope at maximum.
6. Spiked epidermis: chitinous shards and spikes rip out of the character's epidermis, which also hardens into overlapping plates and takes on a glossy sheen of any colour. The shards cause D4 damage to any creature making a melee attack against the character, and the overlapping plates add +1 to the overall constitution ability of the character. In addition, the spikes can be fired off for D10 piercing damage to all surrounding creatures, range 30 ft. The spikes then regrow after D20 hours.
7. Vaporous form: the character seems to be constantly giving off a wispy vapour that trails around them, forming from patches of the character's body that seem to be melting off. The character gains a +1 bonus to their constitution and gains +10 ft of flying movement as they alter the density of their body to a more lighter, gaseous form. The vapour has no other effect and vanishes rapidly if collected.
8. Visage of undeath: the skin and muscle surrounding the character's bones sloughs away in certain patches, giving them a horrifying appearance. At certain points where the bone pokes through, it appears to be horrendously deformed, taking on minor characteristics as seen on the bone structures of creatures from the Far Realm. No other visible changes are made. The character gains +1 to their constitution and gains resistance to necrotic damage.

Note: all effects can be cured through the use of a greater restoration spell. You may add additional small cosmetic effects to your character if you so wish, provided that they make sense in the context of the 'feat'. Optional rule: every character under the effect of the feat takes an 'alignment shift' 1 unit from good to evil and/or 1 unit from lawful to chaotic. E.g. A lawful good character becomes tempered neutral (true neutral) and a neutral evil character becomes chaotic evil.