The fact that Dwarf legend Grombrindal would be appearing in Total War: Warhammer in some capacity leaked some time ago, but he’s now official. Initially available only through special Steam keys acquired from Games Workshop, he’ll eventually be available for everybody in 2017.
Here’s the Steam DLC page for Grombrindal, running down his abilities and the various methods of getting hold of him. Your first option is to get hold of Issue 3 (November 2016) of the re-launched White Dwarf magazine. They’ll all be coming with Grombrindal Steam keys (and a voucher for 20% off Total War: Warhammer – the base game, sounds like, not DLC).
The other option is to visit an actual Games Workshop store from 28 October onwards and ask a member of staff in there for a Grombrindal Steam key card. Those cards will be around until January 2017, or when stocks are depleted; whichever happens first. At some point in 2017, he will “eventually become available for all players … without the need for a Steam key”. It doesn’t commit to whether he’ll be free or not.
There’ll be spare keys and codes for Grombrindal floating around online too, I’m sure. In game terms, he’s a Legendary Lord with the following traits and abilities.