After a lot of fairytale characters take over his swamp, the xenophobic ogre must save the princess to get his swamp back.

Shrek

Solid attempt, however, you might want to avoid using the term 'a lot'
I would say drop 'a lot' altogether or use a more interesting word, like Gaggle.

"After fairytale creatures overrun his swamp, a reclusive ogre must save a princess to get his swamp back."

"After a gaggle of fairytale creatures overrun his swamp, a reclusive ogre must save a princess to get his swamp back."