It's the spell to summon ice. Ice moves from Strange's hands across the street like he's Queen Elsa or something like that. The ice is a solid half an inch thick: thick enough to provide a good reflection.

Standing up, Strange looks particularly smug and happy with himself. "Come, stupid tree," he says, offering his hand to Syrlya, blissfully unaware that Syrlya is going to murder him in his sleep after all this is done. "You're either coming with me or going back yourself."