Siona, Captain of the Pyleas

If an Aura becomes attached to Siona that causes another player to gain control of it, that player will control the triggered ability that creates a Soldier.

Siona's last ability triggers both whenever an Aura enters the battlefield attached to a creature you control and whenever an Aura on the battlefield attached to a different object becomes attached to a creature you control. It doesn't trigger if an effect tries to attach an Aura to the creature it already enchants.

If an Aura becomes attached to a noncreature permanent you control that causes it to become a creature, Siona's last ability triggers. If an Aura becomes attached to a creature you don't control that causes it to become a creature you do control, Siona's last ability triggers. If an Aura becomes attached to a creature you control that causes it to no longer be a creature or to no longer be under your control, Siona's last ability doesn't trigger.