Infinite loop that should need mana but doesn't
The opponent had Elas Il-Kor, Danitha Benalia's Hope with a Necrogen Communion enchant, and Fleshtaker.
So he sacrifices Danitha, who's brought back by Necrogen, and she brings back Necrogen on herself. This triggers Elas' life gain and damage in an infinite loop.
"All good so far", leaving aside any opinions on infinite loops themselves.
However, to do this he uses Fleshtaker's sacrifice ability, which costs 1 uncolored mana. All his lands are tapped, he has no other way to gain mana.
So this shouldn't be possible, and is therefore a bug. A game-breaking one since it kills the opponent in 1 turn at no cost, not to mention the infinite life gain for the perpetrator.
EDIT : the artifact allows mana gain if you tap a legendary permanent, however they're not tapped, and bringing Danitha back from the graveyard without Haste shouldn't allow tapping it unless I'm mistaken. If it does, well, that's messed up and not mentioned anywhere on Necrogen.
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[Deleted User] commented
The Relic is the card with the mana ability, and that ability is unaffected by summoning sickness of the creature that's being tapped for it. So yes, he CAN tap the returning Danitha for the Relic to produce the mana nesessary to loop.