Jerren, Corrupted Bishop // Ormendahl, the Corrupter

{2}{B}Legendary Creature — Human Cleric // Legendary Creature — Demon

Whenever Jerren enters or another nontoken Human you control dies, you lose 1 life and create a 1/1 white Human creature token.

{2}: Target Human you control gains lifelink until end of turn.

At the beginning of your end step, if you have exactly 13 life, you may pay {4}{B}{B}. If you do, transform Jerren.

Flying, trample, lifelink

Sacrifice another creature: Draw a card.

Flying. This creature can't be blocked except by creatures with flying or reach. · CR 702.9

Lifelink. Damage dealt by this also causes its controller to gain that much life. · CR 702.15

Trample. Excess combat damage beyond what's lethal to blockers can be assigned to the player or planeswalker it's attacking. · CR 702.19

Multiple instances of lifelink are redundant, so giving lifelink to a Human that already has it gives no additional benefit. (2021-09-24)
Jerren's last ability will trigger only if you have exactly 13 life at the beginning of your end step. You will be able to pay {4}{B}{B} only if you have exactly 13 life as that ability begins to resolve. If your life total isn't 13 at that point, the ability will have no effect. You won't be able to pay {4}{B}{B} and Jerren won't transform. (2021-09-24)
If, while activating mana abilities during the resolution of Jerren's last ability, you somehow gain or lose life, you will still be able to finish paying the cost and Jerren will still transform. (2021-09-24)