Ward of Bones

Each opponent who controls more creatures than you can't cast creature spells. The same is true for artifacts and enchantments.
Each opponent who controls more lands than you can't play lands.

Crawlspace

No more than two creatures can attack you each combat.

Death's Presence

Whenever a creature you control dies, put X +1/+1 counters on target creature you control, where X is the power of the creature that died.

Compelled Duel

Target creature gets +3/+3 until end of turn and must be blocked this turn if able.

Homicidal Seclusion

As long as you control exactly one creature, that creature gets +3/+1 and has lifelink.

Deadly Wanderings

As long as you control exactly one creature, that creature gets +2/+0 and has deathtouch and lifelink.

Balancing Act

Each player chooses a number of permanents they control equal to the number of permanents controlled by the player who controls the fewest, then sacrifices the rest. Each player discards cards the same way.

Dueling Grounds

No more than one creature can attack each combat.
No more than one creature can block each combat.

Birthing Pod

({G/P} can be paid with either {G} or 2 life.)
{1}{G/P}, {T}, Sacrifice a creature: Search your library for a creature card with mana value equal to 1 plus the sacrificed creature's mana value, put that card onto the battlefield, then shuffle. Activate only as a sorcery.