Crystal Storm

It throws a wide, fast stream of shards that slow down as they travel and wink out shortly after. They bounce off everything and never pierce, so each shard is one hit and then it is loose in the room hunting for something else. Per shard damage is tiny, which is the design: you are filling a space rather than aiming at a point.

Enclosed spaces, and only enclosed spaces. In a corridor or a walled arena every miss comes back and keeps working, and your real damage is far above what the same weapon does firing into open sky. At long range the shards have already slowed to a crawl by the time they arrive, so a distant single target is the worst case for it.

The fire rate is the strength, so anything flat per hit is worth more here than a percentage. Softening a target with Ichor first and then holding the button converts a lot of small hits into a real number. It is the Hallow entry in a set of three tomes that also covers the Corruption and the Crimson, and it is the one built on volume instead of a debuff of its own.