If I become the surprise defender in a showdown and that showdown ends in a combat draw, then what will happen?

According to the rules of Riftbound and the RiftJudge FAQ #4553, if a defender uses an effect to join a showdown (often called a surprise defense) and the showdown results in a draw (meaning both players have units remaining at the battlefield after combat), the attacking units are recalled to their base, and the defender remains to establish control and conquer the battlefield.

Sequence of Events:

1. Surprise Defense: You use a card or ability to move your unit to a battlefield already occupied by an attacker, making you the defender.
2. Combat Resolution: The showdown proceeds through combat.
3. Combat Draw: If a "draw" occurs (e.g., neither side's units are wiped out), the following steps occur during the Combat Cleanup (Rule 461.1.a):
   • Heal all units.
   • Recall all attacking units (since defending units are still present at the battlefield).
4. Result: Because the attacking units have been recalled, you are the only player with units remaining at the battlefield.
5. Control: You establish control of the battlefield. If you have not yet scored this battlefield during your turn, this results in a Conquer.

This interaction holds true because the rule for recalling attacking units on a tie does not require the defender to have been in control beforehand; it only requires that a defender is present at the battlefield after combat damage has been dealt.