Patch 3.6 brought with it visual updates to Trundle and Sejuani and also a new hero, Lissandra of the Freljord. She’s a powerful mage who has a number of stun abilities and a passive that turns hits into free spell casts.
It would be well worth learning her ropes.
Lissandra’s pitched to be a high damage-dealing mage, specialising in crowd control. I know that because we’re told in the champion spotlight video embedded below.
Her passive ability, Iceborn, could potentially be quite powerful, though its unpredictability does hinder its usefulness. It works by occasionally providing a free spell cast, at which point it runs a cool down which is reduced by a second for each enemy Lissandra hits with a movement hindering spell. In a packed battlefield Iceborn could make Lissandra devastating, throwing out free spells frequently, unhindered by depleted mana.
The rest of her moveset is a little dry, Ice Shard is essentially a magic missile that can pass through a line of enemies. Ring of Frost is an area of attack spell which freezes enemies in place (which would partially restore her Iceborn cooldown). Glacial Path is much like Ice Shard, though, during the attack, it allows her to teleport to any point along its line of effect.
Her ultimate, Frozen Tomb, can be targeted on enemies and herself. On an enemy it stuns and freezes them while also creating a wide circle of ice damage around the target. Casting on herself creates the same area of damage but temporarily turns Lissandra invulnerable and makes her untargetable.