There are several grafts and symbionts, that a creature can have and gain powers from. The three fiendish symbionts (there are also psionic symbionts used by creatures like mind flayers, aboleths, or yuan-ti, and one example of a undead symbiont) are the Soul Tick (fine LE), Gutworm (Tiny CE), and Fiendish Familiar (diminutive NE).
Of them, only the soul tick has an attack form. All can telepathically communicate with their host, if said host has a language. They are outsiders and thus have outsider traits.
The fiendish familiar looks like a small shapeless face on a lump of skin, that you graft somewhere in place of actual skin. They help spellcasting hosts by teaching them spells and improving the relevant ability scores by 2 for purpose of bonus spell slots.
The gutworm is an abyssal tapeworm that swims freely in the host's digestive track. (It must tickle.) Gives a inherent boost to Constitution, and can incite a barbarian-like rage in the host at will, without causing fatigue in the host. Once per day, they can neutralize ingested poison in their host.
Soul tick feed on blood and thus can attack with a blood drain. They "permanently" diminish the host's Con score by one, since they'll drain one point of Constitution every day to feed. They can also drain up to 1d4 points as a full-round action. They automatically empower spells with the Death or Evil descriptors, for free. But casting Chaos or Good spells are impeded. The host is continuously protected by a protection from good or from chaos, at the tick's choice, which can change the effect at will as a free action. The host is also considered evil for effects such as holy smite or holy weapons regardless of actual alignment.