Exquisite Blood

Whenever an opponent loses life, you gain that much life.

Cleric Class

(Gain the next level as a sorcery to add its ability.)
If you would gain life, you gain that much life plus 1 instead.
{3}{W}: Level 2
Whenever you gain life, put a +1/+1 counter on target creature you control.
{4}{W}: Level 3
When this Class becomes level 3, return target creature card from your graveyard to the battlefield. You gain life equal to that creature's toughness.

Scoured Barrens

Scoured Barrens enters the battlefield tapped.
When Scoured Barrens enters the battlefield, you gain 1 life.
{T}: Add {W} or {B}.

Marauding Blight-Priest

Whenever you gain life, each opponent loses 1 life.

Faceless Agent

Changeling
When Faceless Agent enters the battlefield, seek a creature card of the most prevalent creature type in your library.

Clawing Torment

Enchant artifact or creature
As long as enchanted permanent is a creature, it gets -1/-1 and can't block.
Enchanted permanent has "At the beginning of your upkeep, you lose 1 life."

Idyllic Tutor

Search your library for an enchantment card, reveal it, put it into your hand, then shuffle.

Revenge of Ravens

Whenever a creature attacks you or a planeswalker you control, that creature's controller loses 1 life and you gain 1 life.

Vito, Thorn of the Dusk Rose

Whenever you gain life, target opponent loses that much life.
{3}{B}{B}: Creatures you control gain lifelink until end of turn.

Soul Warden

Whenever another creature enters the battlefield, you gain 1 life.

Fateful Absence

Destroy target creature or planeswalker. Its controller investigates. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")

Authority of the Consuls

Creatures your opponents control enter the battlefield tapped.
Whenever a creature enters the battlefield under an opponent's control, you gain 1 life.

Swamp

Ondu Skyruins

Ondu Skyruins enters the battlefield tapped.
{T}: Add {W}.

Ondu Inversion

Ondu Inversion // Ondu Skyruins

Plains

Trespasser's Curse

Enchant player
Whenever a creature enters the battlefield under enchanted player's control, that player loses 1 life and you gain 1 life.