Ancestral Katana

Whenever a Samurai or Warrior you control attacks alone, you may pay {1}. When you do, attach Ancestral Katana to it.
Equipped creature gets +2/+1.
Equip {3} ({3}: Attach to target creature you control. Equip only as a sorcery.)

Eiganjo Uprising

Create X 2/2 white Samurai creature tokens with vigilance. They gain menace and haste until end of turn.
Each opponent creates X minus one 2/2 white Samurai creature tokens with vigilance.

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.

Eiganjo Exemplar

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

Akki Ronin

Whenever a Samurai or Warrior you control attacks alone, you may discard a card. If you do, draw a card.

Imperial Subduer

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