Tekuthal does not recieve indestructible counter
Tekuthal, Inquiry Dominus enters as a 3/5 with the ability to gain an indestructible counter. I was able to trigger the ability to gain the indestructible counter in response to an opponents "Mightstone and Weakstone" ability that targetted Tekuthal. The opponent aimed to give me -5/-5, but I paid 1 colorless and 4 life as well as 3 oil counters from mercurial spelldancer to give Tekuthal indestructible. It did not matter, as Tekuthal died anyway. Tekuthal should have gotten indestructible and not died.
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esosia#51500 commented
Indestructible won't save Tekuthal here. The -5/-5 from The Mightstone and Weakstone is neither damage nor an effect reading "destroy target creature".
A creature with zero toughness can't survive as a state-based effect, so it dies even with indestructible.
Had the opponent dealt 5 damage to it, then indestructible would have prevented death. But -X/-X circumvents it by aiming at state-based actions. -
[Deleted User] commented
A creature that has 0 or less toughness is put into its owner's graveyard as a state based action. And this is not destruction, so being indestructible doesn't save it. Destruction happens if a creature with tougness 1 or greater has damage marked on it equal to or greater than its tougness through a different state based action.