Grip of Chaos

If there are multiple legal targets, it may still choose the original one.

If there are no other legal targets, it keeps its target.

The ability triggers when a spell or ability is put on the stack, and this target-changing ability itself goes on the stack. This means that the spell or ability keeps its original target until this ability resolves. You determine the new target when this ability resolves, at which time the set of legal targets may have changed.