These will be found in the regular frame in Set Boosters, with extended-art versions in Collector Boosters
They are legal in whatever formats the cards are already legal in
II, III, IV - Add for each lore counter on this Saga
Aside from being a human, which is a very relevant creature type in Commander, Modern and basically every other MTG format, Esper Sentinel is good because it reads: Whenever an opponent casts their first noncreature spell each turn, draw a card unless that player pays X, where X is Esper Sentinels power. Its a bit like Rhystic Study, but the cost can be far higher if youve got things that make humans stronger or you can put +1/+1 counters onto Esper Sentinel
Depending on who you ask, they can technically mean different things (blinking is end-of-turn return, flicker is instant return) but they're often used in the same way so I'm just going to be bouncing between the two here and there