"There'll be storytelling, fairies, and to top it all off there'll be marshmallows and Milo.
"Come along as your favourite mythical creature or character."
Torches are required, and children need to be accompanied by an adult.
Reweti will be a Patupaiarehe (Māori fairy) on the night, and will be telling "Taniwha" by Robyn Kahukiwa.
Xiong, of Wen's Story Den, is set to tell a traditional Chinese story involving an eyeless dragon.
"It will be a shadow theatre performance," Xiong said.
"Lisa is the narrator, and me, Quinn [Xiong's son] and Ellen are the actors."
As for Keene, she will morph into an "actual fairy" for Wednesday night only, to perform alongside Reweti and Xiong.
"It's taken us a while to tame the unicorn, but he's been sprinkled with fairy dust and has promised to be on his best behaviour," she said.
The unicorn, which belongs to the Red Door Gallery in Putiki, is set to be auctioned for charity after it completes its duties at the museum.
• The show runs from 6pm to 8m on Wednesday, April 21, and is most suited for primary school-aged children. Entry is free, but koha is appreciated.