The latest of Path of Exile's free league updates is called Blight, and it adds tower defense to the action-RPG—a bit like the minigame in The Incredible Adventures of Van Helsing. Tendrils of the fungal Blight lay down paths for enemies to follow, and you'll have to plant towers that fire off elemental effects to exploit their weaknesses. Of course while that's happening you still click on everything until it dies as per usual for the genre.
There's new loot as well, including oils that can be combined by the NPC responsible for those towers, Sister Cassia, and applied to items. There's also an extra skill bar now, as well as various class tweaks and other changes.
Blight's out now, and the Path of Exile website has a whole section devoted to explaining its finer details.