Olivia Voldaren

Flying
{1}{R}: Olivia Voldaren deals 1 damage to another target creature. That creature becomes a Vampire in addition to its other types. Put a +1/+1 counter on Olivia Voldaren.
{3}{B}{B}: Gain control of target Vampire for as long as you control Olivia Voldaren.

Secret Lair Drop

Modern Masters 2017

Innistrad

If Olivia Voldaren deals lethal damage to a creature with its first activated ability, that creature will become a Vampire before dying.

If you activate Olivia Voldaren’s last ability, and before that ability resolves you lose control of Olivia Voldaren, the ability will resolve with no effect. You won’t gain control of the targeted Vampire.