Like usual, you decide to summon a servant

Ooh! Summoning card transform into a red one!

You summoned Tiamat, Class Beast Servant

She's bigger than you

Tiamat: beautiful singing Aaa--Aaa-La~! Stops I'm Mom… No, that's wrong, that's my position. My name is Tiamat. I've come as an Alter Ego. Surprised, humanity? If you perish, my defeat will become meaningless. That's why I felt like helping you out. You're happy, humanity. Good.