EC115 Necrotic Spasms
Attach to a target unit.
At the beginning of its controller’s turn, attached unit takes 1 uncancellable damage.

What's the meaning of "its"? the "Necrotic spasms" card or the attached unit?

Thanks for reply...

The attached unit. The structure of the sentence is clear, and also, when you attach a card to an opposing unit, you're still the controller of the attachment. So if Necrotic Spasm were to refer to its controller's turn, it would just say "At the beginning of your turn ...".