It's less of a grenade and more of a concentrated burst of high concussive force that's localized to a small area--around the size of a human fist. It's designed to blast away armor and travels through solid mass very well, but doesn't do so good while traveling through air, so a cave-in shouldn't be a concern--unless the thing proves to be fast enough to evade one of the lasers, it misses, and hits the catacomb walls. Oh, what a terrible thing to happen. (Tyzias would actually feet super guilty for this.)
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Sudden cave ins do make for good drama, but naturally something characters may try to avoid icly.
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