Catalyst

Full title: Catalyst the Virulent Leviathan

Tendrils can morph into various weapons (scythe, ax, etc) or just attack as tendrils. Tendrils can appear anywhere there is slime! As the name suggests, Catalyst can force living and unliving things to go through disastrous chemical changes, while he is not affected at all.

Virulent = toxic. His spit and blood are toxic to most things.

Other traits: serrated claws, jagged teeth and extended maw, long tongue, living tail - a giant mouth with equally jagged teeth, ready at all times to defend its master.