Angry Nimbus

No item summons it. It spawns on its own in Hardmode whenever rain is falling on the surface, in every biome except the Snow, and the intensity of the rain makes no difference. A light drizzle counts. It will not appear underground, so any farm has to be built above ground and out in the weather. Only two can be alive in the world at a time, which sets a hard ceiling on how fast that farm runs.

It parks itself directly over your head, roughly ten tiles up, and drips damage down on you the way the Nimbus Rod and Crimson Rod do. Melee mostly swings at empty air while you take chip damage trying to close, and it moves quickly and shrugs off knockback on top of that. A fast-firing ranged weapon like the Megashark suits it far better, since you can keep hitting it while you walk out from under the rain.

The cleanest counter is the Slimy Saddle. Get above the cloud and bounce on its head, and it has no way to hurt you at all while you do it.

Because the population cap is two, killing them quickly matters more than gathering a crowd. A rainy Blood Moon is the best window you will get, since it stacks a high spawn rate on top of the storm. Build a firing position with a roof on it that you can shoot upward from, otherwise you spend the whole storm being rained on by the thing you are trying to kill.

A nimbus is an old name for what is now classed as a nimbostratus, the low uniform gray cloud that produces steady rain or snow. The lumpy shape is closer to a cumulonimbus, whose name comes from the Latin for heap and rainstorm.