Basically, the gribbly stuff comes in two flavours. The demonic, which is of the same "stock" as Hellboy, are all horns and baroque imagery, are the denizens of Hell. The "frogs" are the spawn of the Ogdru Jahad, the Seven Who Are One, who are "Lovecraftian" horrors. All tentacles and compound eyes. One of the various gribblies in their thrall (The Ogdru Hem) is The Conqueror Worm, which is what it sounds like - A giant worm whose breath warps and mutates humans into horrific lizard-esque things. It was the physical form of a weird space jellyfish thing.

Come to think of it, a Tzeentch army with a nautical, jellyfish theme could work.