Unsure What's Wrong with Cactus

[cactusincalifornia]

I've had this cactus in my room since last September. I keep it in the corner of my room where it gets a moderate amount of indirect sunlight. The one cactus hidden furthest in the corner (thus receiving the least amount of sunlight) turned black and started wilting. I'm not sure how long ago as it is kind of hidden but it's probably been like that for only a few days. Does anyone know why it looks like this and if there's anything I can do to fix it? Thanks in advance

[Aloinopsis]

This is not a cactus. It is a Euphorbia.

The black part has died from having too little light, too much water, or both.

These plants need warmth and light all year in my experience. They can't really take a cool winter like many cactus can.

[cactusincalifornia]

Thank you for the advice