2a weapon that also breaks your fall
Hold it and the canopy opens over your head, dropping you gently and cancelling fall damage outright. That makes it a real early answer to a deep shaft: dig straight down, hold it out, land safely at the bottom. Tapping down closes the gap faster when the drift gets tedious. It is also a shortsword, and a better one than the plain umbrella, though shortswords are still a poor way to fight anything that fights back.
Swinging it in midair folds the canopy for the length of the attack, so you fall at normal speed while you attack. Do not try to fight and float at the same time over a drop.
stacking and buying
Featherfall runs on a separate system, so the two do apply together, but the umbrella barely changes your descent once the potion is up. Keep it for the trips where you have no potion left.
The Clothier only lists it while he is standing in a Graveyard, which means letting gravestones pile up near one of his houses first. The black canopy and the name both point at the umbrella an undertaker holds over mourners when it rains.