If I attack with Stellacorn that has a buff and am going into showdown against Master Yi Tempered and Gemhand Hunter, and then I play a spell to stun the Yi, does my Stellacorn come back to base exhausted or still ready?

To determine the state of your Stellacorn when it returns to base, we must look at how combat and recalls function in Riftbound.

The Ruling

Your Stellacorn will return to your base in the same state it was in when it was recalled.

According to Rule 453.1 and the RiftJudge FAQ #9378, recalls do not inherently change the state of a unit. Unless a card effect specifically states that a unit is recalled "exhausted" or "ready," it retains its current status (Ready or Exhausted) from the battlefield.

Breakdown of your Scenario

1. Moving to Attack: To initiate a showdown, your Stellacorn must move to the battlefield. Typically, a "Standard Move" requires exhausting the unit as a cost (Rule 414.3.a). Assuming you exhausted your Stellacorn to move it to the battlefield, it is currently Exhausted.
2. Playing the Stun Spell: You play a spell to stun Master Yi. This creates a chain. Once the chain resolves and the stun is applied, Master Yi becomes Stunned.
3. Combat Resolution:
   • If your Stellacorn is stunned or if it loses the showdown, it will be recalled to your base.
   • Because it was Exhausted when it moved to the battlefield, and no part of the recall process changes that state, it will return to your base Exhausted.

Summary: The act of being recalled does not "ready" a unit. If your Stellacorn was exhausted to move into the battlefield, it will remain exhausted upon returning to your base.

Note: This is my interpretation based on the standard rules for movement and recalls. No official FAQ exists for this specific combination of cards, but the interaction follows the established rules for unit states during recall (Rule 453.1).