Selkie was all he was taking with him. His goal wasn't a fight, it was an assassination. Selkie confirmed that Undine had been the Queen's successor. Her only successor as far as she knew. That meant they only needed to kill one person. Once through the Rainbow Bridge Selkie and Sael would go after the queen. With Selkie's ability to shrink down, even smaller than she already was. Combined with the stealth ability he had inherited from Nee he believed he really could pull it off. With his mana topped off thanks to donations from a number of those left behind his plan was to simply hit them hard and fast.
There were complaints of course. "Then why did you have a team put together?" Erica snapped, uncharacteristically loud.
