I control a sin prodder, at my upkeep I reveal a wurmcoil engine, my opponent chooses to take 6 from sin prodder. I then cast my rakdos lord of riots. Can I cast, say, a steel helkite for free? Also can you state where in the specific rules it says?
Your creature spells will cost 6 less to cast this turn, so you can cast Steel Hellkite or another Wurmcoil Engine for free.
Rakdos knows how much life your opponents have lost this turn, including the life that was lost before he entered the battlefield. This isn’t explicitly spelled out in the rules anywhere, but rather it’s just a straightforward reading of his ability.
If Rakdos only cared about life your opponents lost while he was on the battlefield, he’d have a very different ability, probably something like “Whenever an opponent loses life, creature spells you cast this turn cost X less to cast where is the amount of life that opponent lost.”
