The Talisman Challenge Leagues, for which you can check out the trailer above, are split into Standard and Hardcore for players looking for different levels of challenge, each with separate economies and sets of Talismans to find. You get Talismans when you defeat monsters under their influence, and they boost your abilities.
You can also sacrifice sets of Talismans from one tier to summon a monster that's possessed by a Talisman from the next tier. When you sacrifice a set from the third tier, you'll open a portal to the Den of the Wolven King, Rigwald, who's possessed by the most powerful Talisman. Here's how Grinding Gear describes the Talismans:
These leagues contain 32 challenges, and for each eight you complete you get a piece of Rigwald's armour set. You also collect pieces of the challenge Hideout Totem.
The eight new skill gems include three bow skills (Shrapnel Shot, Blast Rain, and Siege Ballista), two physical damage spells (Blade Vortex, and Bladefall), and three chaos damage spells (Contagion, Essence Drain, and Wither). You can probably guess what the spells do from their names, but check out the full post for more info on those and the ways the new support gems work.
The update will be available on December 11, and more patch notes are on their way, as well as details of the next major expansion for Path of Exile, due in early next year.