Unknown has encountered a trigger that has unlocked his ice magic, as well as a trigger that has unlocked energy manipulation.
To be more specific, these triggers were seeing someone use ice magic, and seeing someone use energy manipulation, in that order, which occurred during a scene in the bar. Devyn conjured a sort of ice-based fire, and Victoria Midnight used energy manipulation to control his own spell. Seeing these arts being used unlocked memories "Unknown" had of them, and when he gave them deeper thought, he realized his own potential.
His Ice Magic ranks 6/10 on his character page, which puts him above average in skill level. To manipulate ice requires two things - water, and an energy transference. When the ice is created, it can be directed and controlled. Ice Magic is a subset of water magic.
His Energy Manipulation ranks 8/10 in his character page, which puts him at a highly adept skill level. With this magic, energy can be manipulated but not created. "Unknown" can only control pre-existing energy, and what kinds of energy he can manipulate is rather limited. He also has limits to how much energy he can handle at once, as does anyone. Energy Manipulation is more or less a requirement for conjuring ice.