Any field marked "???" is one I haven't come up with something for, suggestions are welcome
Series 1 (Mini Totem Series):
Summary: A series that isn't as damaging as, say, the Blast Bomb series but has basically zero knockback.
Appearance: A miniature Grim Totem with a few mechanical bits attached.
Bomb Effect: Dozens of zombie arms burst forth and swat at opponents in the AoE.
Backstory: A failed attempt by the Spiral Knights to repurpose the Grim Totems as a revival system.
Names:
2-star: Horror Headstone
3-star: Grim Grave*
4-star: Warped Ward
5-star: Terror Totem
Series 2 (Kat Kage Series):
Summary: A shadowy shard bomb, basically.
Appearance: A Cat cage with a ghostly aura.
Bomb Effect: Dozens of Ghostly Kats burst forth from the cage.
Backstory: Originally used by the Knights to release captured Kats after trying to study them.
Names:
2-star: Kat Kage
3-star: Pet Cemetery*
4-star: Feline Ferocity*
5-star: ???
Series 3 (Dark Blast Bomb Series):
Name says it all.
Backstory: Blast Bombs imbued with dark energies.
2-star: Dark Blast Bomb
3-star: Shadow Blast Bomb
4-star: Midnight Blast Bomb
5-star (Fast/Normal): Nightshade Bomb
5-star (Slow): Grimalkins Fury
"Grimalkins Fury" has a different effect and such to the other bombs:
Appearance: Resembles a more Skull-like BAB with a spectral aura.
Bomb Effect: A Grimalkin bursts forth with a shockwave and a roar and then disappears.
Description: A Bomb imbued with the wrath of a mighty Grimalkin, only the bravest of Knights dare carry something so dangerous.
Possible Extra: All nearby enemies get the "Panic!" effect as it explodes just like the Knights do when a regular Grimalkin gets near.
* Name suggestion courtesy of Coneykrab
Perhaps for the 4*-5* version of the bomb, a good goal for the animation COULD be something like in this Youtube video (game is Tales of Vesperia and the animation is basically for the attack "Violent Pain" which is what I thought of with your bomb description. Mostly because it would actually look vaguely like the energy given off when you revive yourself, but then with the "gone wrong" idea behind it, the energy could collapse back on itself in that manner and give purpose for how the bomb actually inflicts the damage.
http://www.youtube.com/watch?v=zg3rYgaLtZk&t=2m32s
That particular attack would be a good "Grim" visual animation, but as you said, replace the shadow animation with zombie arms and each arm doing a bit of damage when it comes into contact with a target or targets, should there be more than one caught in the blast.
Otherwise I'm kind of lost on what to suggest for anything else, but I'm interested in seeing where this goes.