When Kroxa enters the battlefield, it triggers twice, as well as Selvala.
Activate Selvala by paying and tapping it, adding 3.
Resolve the first Kroxa trigger, causing you to sacrifce it.
When Kroxa dies, Selvala and Mortuary trigger.
Resolve the Selvala trigger, untapping it.
Resolve the Mortuary trigger, putting Kroxa on top of your library from your graveyard.
Resolve the second Kroxa trigger, causing each opponent to discard a card and then lose three life if they didn't discard a nonland card.
Resolve the Selvala trigger from step 2, causing you to draw Kroxa from your library.
Repeat.
This clever loop puts Kroxa, Titan of Death's Hunger through an endless cycle of sacrifice and rebirth, turning Selvala, Heart of the Wilds into a massive mana engine that effortlessly fuels itself. Whenever Kroxa hits the battlefield, its inevitable sacrifice triggers Mortuary to place the legendary Titan right back on top of your library instead of letting it rest in the graveyard. Selvala, Heart of the Wilds happily keeps pace with the revolving door of creatures, her mana-producing tap ability feeding off Kroxa's high power while Thornbite Staff's automatic untap trigger ensures she stays ready to fire off more green and colored mana. As Kroxa continuously enters and leaves the battlefield over and over again, Selvala draws the card you just stacked, letting you replay the threat to drain your opponents' life to zero.