Bertrams Quasit

Shapechanger. The quasit can use its action to polymorph into a beast form that resembles a bat (speed 10 ft. fly 40 ft.), a centipede (40 ft., climb 40 ft.), or a toad (40 ft., swim 40 ft.), or back into its true form. Its statistics are the same in each form, except for the speed changes noted. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Magic Resistance. The quasit has advantage on saving throws against spells and other magical effects.

Bertrams Familiar. Once this quasit has been "baptized" by its master, it becomes Bertram's familiar: It supports its master as long as the master does not violate its will. However, the familiar listens to the wishes of its master and tries to fulfill them (played by the DM). In combat, it behaves cautiously (invisible or shapeshifting) and might be affected by AoE. Although the quasit does not level up in the traditional sense, it can acquire skills and improve existing stats after consulting with the DM.

Claws (Bite in Beast Form). Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d4 + 3) piercing damage, and the target must succeed on a DC 10 Constitution saving throw or take 5 (2d4) poison damage and become poisoned for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Invisibility. The quasit magically turns invisible until it attacks or uses Scare, or until its concentration ends (as if concentrating on a spell). Any equipment the quasit wears or carries is invisible with it.

Bertram is MY Bro!. At its master's telepathic command (bonus action / 1 per long rest), the quasit (however in which form) lunges at a specific target within its range, distracting it with savage insults and irritating grimaces. The target must succeed on a Charisma saving throw (*) or its next action is with disadvantage. *DC = 8 + PB (Bertram) + Cha. Mod. (quasit)