Wondrous Item, Weapon (Longsword), Very Rare, Requires Attunement
This silver longsword counts as a +1 magical weapon, granting a bonus of +1 to all attack and damage rolls made with it. A silver weapon has no unique features against most targets, but some fiends, undead, and other such creatures, with resistance to slashing damage, will be susceptible to this weapon. Some creatures will even take additional damage from this weapon.
This sword also possesses the soul gem responsible for its enchantment fixed within the crossguard. Its magic grants boosts to your natural abilities, granting you the following features:
Extra-Planar Constitution: The magic of the soul gem grants the attuned the ability to call upon the power of the soul bound within, granting resistance to one of your current Mutagen's negative effects for 1 minute. As a bonus action, you can activate this feature a number of times equal to half your proficiency bonus (rounded down) per day.
Lelele: A number of times equal to half your proficiency bonus per day, as an action, the attuned creature can make a weapon attack using Mutant's Ally against all creatures of the user's choice within 10ft. Make a separate attack roll for every viable target, and deal damage to all targets successfully hit.