Embercleave

Flash
This spell costs {1} less to cast for each attacking creature you control.
When Embercleave enters, attach it to target creature you control.
Equipped creature gets +1/+1 and has double strike and trample.
Equip {3}

Avatar of Slaughter

All creatures have double strike and attack each combat if able.

Holga, Relentless Rager

Haste
Holga, Relentless Rager must be blocked if able.
Whenever Holga attacks, each creature you control attacking a player gets +1/+0 until end of turn for each creature that player controls.

Blast-Furnace Hellkite

Artifact offering (You may cast this spell as though it had flash by sacrificing an artifact and paying the difference in mana costs between this and the sacrificed artifact. Mana cost includes color.)
Flying, double strike
Creatures attacking your opponents have double strike.

Assault Strobe

Target creature gains double strike until end of turn. (It deals both first-strike and regular combat damage.)

Chaos Terminator Lord

Lord of Chaos — At the beginning of combat on your turn, another target creature you control gains double strike until end of turn.

Berserkers' Onslaught

Attacking creatures you control have double strike.

Blood Mist

At the beginning of combat on your turn, target creature you control gains double strike until end of turn.