If Evra isn't on the battlefield when its activated ability resolves, the exchange can't happen and the ability will have no effect. However, if Evra is on the battlefield but has power 0 or less, the exchange happens and you'll lose the game.

When its activated ability resolves, Evra's power will become your former life total and you will gain or lose an amount of life such that your life total equals Evra's former power. Other effects that interact with life gain or life loss will interact with this effect accordingly.

Any power-modifying effects, counters, Auras, or Equipment will apply after Evra's power is set to your former life total. For example, say Evra is enchanted with Dub (which makes it 6/6) and your life total is 7. After the exchange, Evra would be a 9/6 creature (its power became 7, which was then modified by Dub) and your life total would be 6.