https://www.vekn.net/forum/rules-questi ... s-rulings/
Ankha wrote: You cannot play Eternals of Sirius if you cannot pay its cost, for instance if you are at 3 pool (Eternals of Sirius costs 4 pool).
You can play Eternals of Sirius if you are at 4 pool: you pay the card and gain the 5 pool at the same time, leaving you alive at 5 pool.