Ideas for Promethium Grenades:
Ranged:
--6" Range, Small blast, str 4 ap 4, scatter d6.
Model under hole is hit by AP 2. Inititative of target squad is reduced by 1 for the remainder of the player turn (for consequent assaults), because they're on fire.
--6" Range, throws the FLAME TEMPLATE. Str 4 ap 4 (or 5), scatter d6, point in a random direction.
Initiative of target squad is reduced by 1 for the remainder of the player turn, because they're on fire.
-probably limit the squad to 5, or allow few characters with grenades.
ASSAULTS: yes, I think they should use firebombs in combat!
--Much like the Space Marine Tyranid Hunters, they could sacrifice their normal attacks for a single Str 6 AP 3 hit, as they stuff the grenade down their opponent's mouth or ear/ down their armor.
--Any number of models with Promethium Grenades can activate their grenade in combat and lob it through the melee. Rather than attacking, place the small blast template d6" from the thrower. Hits are resolved at Str 4 AP 4. Note this can (and probably will) affect friendlies.
-- ONCE during the game, any number of models with promethium grenades can choose to activate all of them at once, though this is sure to end in that hero's death. Place the small (maybe large?) blast template over the explodee. All hits are resolved at str 8 ap 3. Remove the model as a casualty, no saves of any sort are allowed.
Other rules:
Promethium Grenades count as Frag Grenades for the purpose of assaulting into cover, though if they do this, the unit cannot attack with their special grenade CC rules.
When lobbing grenades from an elevated position (area 2 terrain or higher ?), the range of the grenades is increased to 12", though they still scatter d6.
Promethium Grenades ignore the effect of cover.
Anyway, these are some rules I had fun messing around with. The points of your marines may or may not have to change with these. Enjoy!
