Hi! My alocasia has suddenly started losing lots of leaves and is now developing dark patches on its remaining ones.. Any idea what this may be?

It looks like cold damage

A little more history, and pictures including the pot, might add to the story and answers.

Yes, please, members here would need a little more care info.

How long have you had your plant?
Has it been repotted? If so, how long ago?
Does the pot have drainage holes?
How often is it watered? How much and how do you decide when to water?
How close to the nearest light source/window?
Did you have it outside recently or somewhere were it could have been cold damaged?