You die when you're at 0 life, the next time state-based actions are checked. This is a change from previous (albeit very old) rules. In your example, you would die before you could activate Tainted Sigil, long before the ability would resolve.
Note that state-based actions aren't checked during resolution of a spell or ability, so if you temporarily go to less than 1 life, but the same ability or spell puts you back to 1 or more, you don't lose the game.
April 6, 2016 5:04 a.m.
Quite precisely actually, you can't actually pay 50 with Purge:
117.3. A player cant pay a cost unless he or she has the necessary resources to pay it fully. For example, a player with only 1 life cant pay a cost of 2 life, and a permanent thats already tapped cant be tapped to pay a cost. See rule 202, Mana Cost and Color, and rule 602, Activating Activated Abilities.
You can only pay as many life as you have.
If you were to pay 20 though, you'd die at the spells resolution. After the spell would resolve, state-based actions are checked before anyone gets priority, and you at 0 life would end the game before you can activate the Sigil.
April 6, 2016 5:08 a.m. Edited.
Thanks, that suspicion crept up on me after I posted the question. The main concern however was if I could save myself from dying, turns out the whole scenario was fundamentaly flawed :) . Back to the drawing board.
April 6, 2016 5:14 a.m.
@JohnnyBaggins: You can pay 50 life as long as you have at least 50 life...
April 6, 2016 1:44 p.m.
The number of games in which decks who want to cast spells like Plunge into Darkness happen to have 50 life should converge towards 0, I just assumed the average case where people in eternal formats rarely ever have 50 life :>
April 6, 2016 2:12 p.m.
The idea currently,new knowledge of the rules taken into consideration, is to cast Plunge into Darkness, digging as deep into my deck as my life total allows and fetching Children of Korlis, playing Children of Korlis and sacrificing them to "gain" a lot of life. My life total still stays the same, but other effects that care about gaining life would have a mighty feast. However the fact that I can usually only do this with 19 life (and even that is unlikely) makes the strategy a lot less viable.
(Not that it was viable to begin with; I just love tinkering with somewhat obscure Cards)