Spellwurm Symbiont (5e Equipment)
From D&D Wiki
Wondrous Item, very rare (requires attunement by a spell caster)
This symbiont looks like a long snake like creature the size of a human arm.
While attempting to attune to the Spellwurm Symbiont the user gains one level of exhaustion, and every time you finish a long rest you take 2d6 psychic damage.
Once bonded, your spell attack rolls, and your spell save DC increases by half of your proficiency bonus rounded.
To unattune from the spellwurm, the user must make a DC 15 Constitution saving throw. Regardless of whether the user succeeds, they take 2d8 psychic damage.