Its a Reserved List card, remember, so its current retail price of $40 might be more permanent than you might think
That's very strong, and many commonly played 3 or smaller toughness Commander Classics are killable with this, like Hate Bears like Esper Sentinel, Goblin Welder, Flametongue Kavu, Eternal Witness, Soul Warden, Ravenous Chupacabra, and loads more
You are specifically casting the back side of a battle when it transforms, after the trigger to transform it once it is defeated resolves
Pumping your Merfolk creatures a bit and granting them a very powerful form of evasion with islandwalk means you are free to keep attacking so long as your opponent controls an island
(As of this writing, the last one wont show up
Right now, one of Amethyst's most recognizable features is cards that bounce back to your hand or the battlefield after they've been removed (something other trading card game communities call Recursion)