If I'm at a battlefield with a mutated mouser unit and I charm an enemy sharkling unit into that battlefield, who is the attacker and who is the defender?

When you use Charm to move an enemy Sharkling into a battlefield where you already have a Mutated Mouser, the Sharkling becomes the attacker and you (the controller of the battlefield where the Mutated Mouser is located) become the defender.

This interaction follows the standard rules for contesting battlefields, as clarified by several RiftJudge FAQs:

• Attacker/Defender Designation: As confirmed by RiftJudge FAQ #5394 and FAQ #2395, the enemy unit moved to contest a battlefield you control becomes the attacker, even though your spell caused the move. It is the Sharkling that is applying the contested status to your battlefield.
• Controller of the Battlefield: You are the defender because you hold the battlefield where the showdown is triggered.

Key Mechanics for this Showdown:

• Attacker/Active Player: Per RiftJudge FAQ #2395, the attacking player (the controller of the Sharkling) becomes the active player once the showdown begins and gains focus and priority first.
• Unit Abilities:
  • The Sharkling will have its [Assault 4] active because it is designated as an attacker.
  • The Mutated Mouser will have its [Shield 2] active because it is designated as a defender.
  • The Mutated Mouser also has [Tank], which means the attacking player must assign combat damage to the Mutated Mouser first during the Combat Damage step (per Rule 460.2.c.5).