Richard lobs the grenade a short distance ahead of the horde and starts following Sean backwards.

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The grenade goes off after they next move, but unless they speed up or down compared to before Richard should have it about right to catch as many of them as possible. It does 16 crushing damage point blank, 5 crushing a yard away, 2 damage at two yards (I figured we'd rather not do the "roll damage for everyone caught in the blast separately" approach given the number of enemies involved - let me know if you'd rather they all get a roll each GM).

The fragmentation suffers a -3 range penalty to hit Richard (he moved 2 yards back, factoring in encumbrance and the whole moving back is 1 yards per 2 normal movement thing), so he gets hit by 2 fragments, both hitting the torso and thus doing nothing against his steel plate. For the enemy, given size and posture mods are both -0 for them, that makes it so those within three yards get hit by 3 fragments, and those further out get hit by 2. That includes any of our people within 10 yards of the grenade, which may include Sean (oops). I'm also not sure if I should've rolled fragmentation damage separately against everyone - given I didn't, and I'm not sure how many will be left when it detonates, I'll again leave that call to you GM.