Saving Grace enters the battlefield and triggers, causing all damage that would be dealt to you and to permanents you control this turn to be dealt to
Crystalline Crawler instead.
Activate
Pestilence Demon by paying ({B} magic symbol) , dealing 1 damage to each opponent and each creature they control, and attempting to deal 1 damage to you and each creature you control.
Saving Grace redirects that damage to
Crystalline Crawler.
Vigor's replacement effect prevents that damage and puts that many +1/+1 counters on
Crystalline Crawler instead.
Activate
Crystalline Crawler's first activated ability by removing a +1/+1 counter, adding ({B} magic symbol) .
Repeat from step 3.
Results
Infinite +1/+1 counters on
Crystalline Crawler.
Infinite colored mana.
Infinite damage to opponents and each creature your opponents control.