Mondrak doesn't die because of -x/-x effect
Mondrak did not die because of the -X/-X effect on bloodtithe harvester.
I had attacked with a Titan of Industry, opponent blocked and afterwards I sacc'ed my bloodtithe harvester to finish the mondrak off. But it did not die and only had -2 tracked on it.
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[Deleted User] commented
Mondrak's toughness was still 4 despite having taken 4 damage. Because damage does not reduce toughness. Arena just shows damage taken as a reduced toughness to make it easier to see how much more damage is needed to destroy it. So a -2 from the Harvester brings the indestructible Mondrak only down to 2 toughness and 4 damage marked, not enough to kill it. You need to reduce the toughness all the way down to 0 or less to kill an indestructible creature. Damage doesn't matter at all.