The Avatar of Khaine
The Avatar is still a pretty nasty monstrous creature. He’s tough in combat, deals good damage, and has a strong though short-ranged shooting attack. With the new Daemon rules, he only has a 5+ invulnerable save. Paired with his 3+ save, that still makes him more durable than most monstrous creatures rampaging around the table now. He makes your units within 12” Fearless, which is great for mitigating the Eldar weakness for failing break tests. For the first time, he has Fleet which really helps him reach the enemy. He also gets Battle Focus so he can close with the enemy and keep blasting with his pseudo-meltagun.
He also has the ability to take Exarch powers, which give him some neat customizability. Most people like Fast Shot so he can shoot the wailing doom twice, but I like Monster Hunter as well in case he comes up against other monstrous creatures (which are very common around here at least). I don’t like Crushing Blow since he can smash if he needs to. Defensively, Disarming Strike allows the Avatar to remove any power fists or Instant Death inflicting weapons from a challenge. Granted, he’ll probably slaughter most challengers, but it could come in handy against some special characters.
While he doesn’t do as much to support other units as our psykers, the Avatar is substantially more threatening on his own. His Fearless bubble provides an excellent anchor for your infantry. I wouldn’t take him as a solo HQ, but he’ll pair well with a less aggressive Farseer or Spiritseer. He would also work well paired with Wraithlords or Wraithknights, as more monstrous creatures will dilute enemy heavy weapon fire.
Autarch
The Autarch is kind of a jack-of-all-trades HQ; he’s not amazing at anything but he can be equipped for a variety of situations. Basically, he comes with good statline (for an Eldar), a 4+ invulnerable save, and can take any piece of basic equipment from any warrior aspect. This allows you to make him to fit most any situation, but a few configurations are most common.
An Autarch on a jetbike with a laser lance is a decent mobile close-combat threat, and can carry a fusion gun for vehicle busting or a reaper launcher for some marine killing firepower. He can join a Guardian Bike squad to give them some assault ability, but he’s at his best paired with Shining Spears or a Warlock Council on jetbikes. If you’re extra daring, you can give him the Mantle of the Laughing God and run him on his own. With a 2+ re-rollable cover save, the enemy will have a tough time killing him before he reaches assault.
The other common build takes a warp jump generator, power weapon, and fusion gun. Add him to a Warp Spider unit to give it much greater ability to kill heavy vehicles, as well as add some assault power. A banshee mask will also help if your Warp Spiders need to charge into combat, as it lowers the initiative of all enemy models even with only one banshee mask.
Never run an warp jump generator Autarch on his own, as it’s likely he’ll get sucked into the warp. I would also avoid giving him wings and joining him to Swooping Hawks, as they’ll lose their Heralds of Victory rule and have to scatter normally when they deep strike.
Beyond his role on the battlefield, the Autarch also allows you to add or subtract 1 from any of your reserve rolls, choosing after each roll is made. This ability alone is worth taking an Autarch if you’re taking an army that relies heavily on reserves. This is most important if you bring lots of flyers, but also will be helpful if you need to keep units in reserve to protect them from enemy fire. I find this particularly helpful for our grav tanks if I have second turn; by keeping my tanks in reserve, I prevent the enemy from shooting them before they can move and get their Jink save.