The composite slime shares many properties with its primal predecessor, the slime. (An ooze, in D&D terms.) One of them is a body composed entirely of an acidic jelly. Because of this, it is 100% resistant to acid damage, and (unlike all other magic types), are neutral to poison. 

Having a highly viscous body and no internal organs has its advantages! Take half damage from all physical attacks. 

Your very existence relies on the power of the arcane artifact that holds you together. The old artifact is fragile, and it's magic idiosyncratic. Under great magical duress, you struggle to maintain composure. You take double damage from all types of magic attacks.

Your bizarre jelly body takes enemies by surprise, and if you have the fortitude for it, you can attempt to hold onto the attackers weapon with your viscous slime body. 

As a reaction, once per short rest, if you are struck with a weapon you may make an opposed athletics check to disarm the attacker. 

While they are seemingly regulated by the arcane core, you still inherit the traits of your bloodline. +2 STR, +2 CON, -2 CHA

A composite slime's centre of mass is not fixed, so losing balance is nearly impossible.  Cannot be knocked prone or stunned.