If I target my opponent's unit at a battlefield with a kill spell, and they react by playing a hidden unit at that same battlefield, does the hidden unit resolve before the kill spell?

Ruling: Yes, the hidden unit enters play before the kill spell resolves. Permanent items (including units) leave the chain immediately after finalizing and enter play, then the chain continues resolving in reverse order.

Sequence:

• Kill spell is played (chain link 1)
• Hidden unit is played as a reaction (chain link 2)
• Hidden unit finalizes, leaves the chain, and enters play at the battlefield
• Kill spell resolves

Nuances:

• Hidden cards gain the "this battlefield" designation when they are hidden and must be used on that battlefield unless it becomes illegal to do so
• This permanent-falling-off-chain rule is why reactive cards like Defy can function properly