The mechanic theyre all derived from is that Morph mechanic
Giovanni lets you get away with stuff you shouldnt get away with and thats what makes it such a dangerous Supporter: your opponent can believe they just managed to escape unscathed and you can crush their hopes with a flash of this card
Double-faced cards will make a big appearance in MTG Final Fantasy too, though this is a returning mechanic as opposed to a new one
Like most Magic: The Gathering cards, the value of Ring of Immortals depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Legends)
You can benefit from playing spells on your opponents turns with cards like Wavebreak Hippocampus and Naiad of Hidden Coves
Five that jump to my mind are Wild Research, Holistic Wisdom, Enduring Renewal, Dross Scorpion and Blinkmoth Urn