Legolas's Quick Reflexes

Instant

Split second (As long as this spell is on the stack, players can't cast spells or activate abilities that aren't mana abilities.)

Untap target creature. Until end of turn, it gains hexproof, reach, and "Whenever this creature becomes tapped, it deals damage equal to its power to up to one target creature."

Players still get priority while a spell with split second is on the stack; their options are just limited to mana abilities and certain special actions.
Players may turn face-down creatures face up while a spell with split second is on the stack.
Split second doesn't stop triggered abilities from triggering, such as that of Chalice of the Void. If one does, its controller puts it on the stack and chooses targets for it, if any. Those abilities will resolve as normal.
Casting a spell with split second won't affect spells and abilities that are already on the stack.
If the resolution of a triggered ability involves casting a spell, that spell can't be cast if a spell with split second is on the stack.
After a spell with split second resolves (or otherwise leaves the stack), players may again cast spells and activate abilities before the next object on the stack resolves.