I'm a bit confused about the ruling regarding cards that say "if the creature would die this round, exile it instead," such as Ob Nixilis' Cruelty. If the target of such a card has the Indestructible state, then shouldn't it follow that even if its toughness falls below zero, it wouldn't die, and since it wouldn't die normally, shouldn't be exiled?
You are correct that an indestructible creature would not be exiled by removal that replaces dying with exile if it could not be killed by said removal. Ob Nixilis's Cruelty though is absolutely capable of killing indestructible creatures, since it reduces their thoughness to/below 0 while indestructible just protects against lethal damage and "destroy" effects.
So, an indestructible creature with toughness 4 or less would not be exiled by a Lava Coil (except it would die as a result of another effect later that turn - the replacement effect would still work in that case).
I understand what you're saying, but I was under the impression that a creature with indestructible cannot properly die, and therefore would not be susceptible to such a conditional statement. Would you happen to have a specific rule I could use as a reference?
Being indestructible only relates to being destroyed, and does not directly relate to dying:
702.12. Indestructible
702.12a Indestructible is a static ability.
702.12b A permanent with indestructible can’t be destroyed. Such permanents aren’t destroyed by lethal damage, and they ignore the state-based action that checks for lethal damage (see rule 704.5g).
702.12c Multiple instances of indestructible on the same permanent are redundant.
and
700.4. The term dies means “is put into a graveyard from the battlefield.”
Ob Nixilis's Cruelty though is absolutely capable of killing indestructible creatures, since it reduces their thoughness to/below 0 while indestructible just protects against lethal damage and "destroy" effects.
So, an indestructible creature with toughness 4 or less would not be exiled by a Lava Coil (except it would die as a result of another effect later that turn - the replacement effect would still work in that case).
W(W/U)U Ephara - Flash & Taxes W(W/U)U || B(B/G)G Meren - Circle of Life B(B/G)G
RGW Marath - Ever shifting Wilds RGW || (U/R)C(W/B) Breya - Artificial Dominion (U/R)C(W/B)
UBR Becket Brass - take what you can, give nothing back UBR
and
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules