Torgaar, Famine Incarnate

As an additional cost to cast this spell, you may sacrifice any number of creatures. This spell costs {2} less to cast for each creature sacrificed this way.
When Torgaar enters, up to one target player's life total becomes half their starting life total, rounded down.

Game Night 2019

As an additional cost to cast this spell, you may sacrifice any number of creatures. This spell costs {2} less to cast for each creature sacrificed this way.
When Torgaar, Famine Incarnate enters the battlefield, up to one target player's life total becomes half their starting life total, rounded down.

Dominaria

Dominaria Promos

For your life total to become half your starting life total (normally 10, half of 20), you gain or lose the appropriate amount of life. For example, if your life total is 4 when Torgaar's ability resolves targeting you, it will cause you to gain 6 life; alternatively, if your life total is 25 when it resolves, it will cause you to lose 15 life. Other cards that interact with life gain or life loss will interact with this effect accordingly.

In a Two-Headed Giant game, Torgaar's ability causes the team's life total to become half the team's starting life total (normally 15, half of 30), but only the target player actually gains or loses life.

Torgaar's first ability can't reduce its cost below {B}{B}. You can sacrifice any number of creatures, even if they won't reduce Torgaar's cost any further.