Peerless Samurai

Menace (This creature can't be blocked except by two or more creatures.)
Whenever a Samurai or Warrior you control attacks alone, the next spell you cast this turn costs {1} less to cast.

Swamp

Isshin, Two Heavens as One

If a creature attacking causes a triggered ability of a permanent you control to trigger, that ability triggers an additional time.

Heiko Yamazaki, the General

Trample
Whenever a Samurai or Warrior you control attacks alone, you may cast target artifact card from your graveyard this turn.

Windcrag Siege

As this enchantment enters, choose Mardu or Jeskai.
• Mardu — If a creature attacking causes a triggered ability of a permanent you control to trigger, that ability triggers an additional time.
• Jeskai — At the beginning of your upkeep, create a 1/1 red Goblin creature token. It gains lifelink and haste until end of turn.

Selfless Samurai

Whenever a Samurai or Warrior you control attacks alone, it gains lifelink until end of turn.
Sacrifice this creature: Another target creature you control gains indestructible until end of turn. (Damage and effects that say "destroy" don't destroy it.)

Norika Yamazaki, the Poet

Vigilance
Whenever a Samurai or Warrior you control attacks alone, you may cast target enchantment card from your graveyard this turn.

Imperial Subduer

Whenever a Samurai or Warrior you control attacks alone, tap target creature you don't control.

Raiyuu, Storm's Edge

First strike
Whenever a Samurai or Warrior you control attacks alone, untap it. If it's the first combat phase of the turn, there is an additional combat phase after this phase.

Asari Captain

Haste
Whenever a Samurai or Warrior you control attacks alone, it gets +1/+0 until end of turn for each Samurai or Warrior you control.