When an opponent responds to a targeted spell (like Hidden Blade) or a unit with a 'When you play me' ability (like Harnessed Dragon) by moving the target, can the target change or does the spell/ability fizzle?

Ruling: For targeted spells like Hidden Blade, targets must be chosen when the spell is played (finalized) and cannot change. If the target becomes illegal when the spell resolves, it mistargets and has no effect. For units with 'When you play me' triggered abilities like Harnessed Dragon, the unit resolves first with no priority window, then the triggered ability goes on the chain and targets are chosen at that point.

Sequence:

• Hidden Blade: Choose target when cast → opponent can respond to move unit → spell rechecks legality on resolution → mistargets if unit is no longer at battlefield
• Harnessed Dragon: Play unit → unit auto-resolves and enters play (no priority window) → triggered ability goes on chain → choose target for trigger → priority window opens where opponent can respond

Nuances:

• Hidden Blade still technically resolves even when it mistargets, it just has no effect (matters for cards like Ravenbloom Student)
• For Harnessed Dragon's trigger, since it has no targeting restriction beyond 'enemy unit', retreating to hand is one of the few ways to save the targeted unit
• The unit permanent and its triggered ability are separate items on the chain