One of the first solos released for the Skorne when Hordes was new, the Ancestral Guardian is a construct that follows Skorne into battle, to preserve the spirits of fallen Skorne warriors, so that they are not sent to the void upon their death.
It's very slow, and has a really low DEF. Not many attacks are likely to miss it. However, it does get a great MAT (of 8!) and it's ARM allows it to be pretty durable as well. With 10 Damage, it should be able to take a hit or two before crumbling. It is 3 points, however, which actually feels a little high for what it does. That's just my opinion though.
Back in the day, this used to have Weapon Master. Sadly, it no longer does. It is a P+S 13 weapon with Reach and it counts as a Magical Weapon. Its strong enough to dish out some damage, and with the mentioned traits, it's not going to have a hard time finding targets one the enemy starts contacting the Skorne lines.
Construct: No fleeing for this guy! Not that he would get far even if he did.
Defensive Strike: Since he's slow moving, he's going to be getting charged in most games. With this, however, the Guardian has a way to counteract it some, by letting him take a free swing at the enemy. Very handy ability, just remember you can't boost your rolls on this attack. Certainly can't complain about a free attack though.
Soul Guardian: The Ancestral Guardian can hold up to three souls, which he can get from the deaths of nearby friendly living faction warrior models, then use those souls during his activation to boost attack or damage rolls, or gain an additional attack. You're going to need to run the Guardian near a unit of some sort, but with Reach he won't be hurting for chances to get into combat anyway. Even in death your Skorne units can serve you. This ability lets the Guardian become fairly intimidating in melee.
Spirit Driven: The Guardian can also spend a soul token at the beginning of its movement to gain another 2" of movement. This allows the guardian to have an 11" threat range- that's not half bad.
Steady: Can't be knocked down. Not that opponents are going to need much help hitting the Ancestral Guardian, but it does mean you won't be deprived of your Defensive Strike.
Master Tormentor Morghoul: No real synergies.
Archdomina Makeda: Walking Death allows the Guardian to get double the use out of your warrior models. They'll die, providing souls for the Guardian, then return, hopefully to die again eventually and provide MORE souls for the Guardian! Carnage allows the Guardian to hit almost anything it wants, especially if it boosts. Savagery allows the Guardian to have a 13" threat, and Defender's Ward makes the Ancestral Guardian even harder to kill (ARM 20!). The spells have much better targets, however, so you'll likely get the most use out of Carnage.
Cyclops Savage: Prescience is pretty nice on the Guardian, since you're likely to be running on only a few souls at a time, and you'll want to use them as efficiently as possible to deal damage.
Titan Cannoneer: No real synergies, but Diminish around the Guardian is helpful for keeping it alive.
Titan Gladiator: No real synergies.
Paingiver Beast Handlers: No synergies.
Praetorian Swordsmen: With the Unit Attachment, I think these guys make a great pair. Swordsmen are cheap, and are likely to die once the fighting starts getting the thickest. With Ranked Attack, the Guardian can stand behind the Swordsmen and get some swings in without taking too much damage in return. Defensive Strike may even net you a free kill over a Swordsmen head to help keep the Swordsmen alive.
Agonizer: No real synergies.
Given that many Ancestral Guardians were sent to the west to assist with the invasion, it's no surprise to see that epic Makeda has them available in her theme force. With all the infantry, he's sure to have a full stock of souls to use.
Lord Tyrant Hexeris can take them as well, and if you get up to Tier 4, the Guardian will actually get another 6" onto the table before the game starts. Helpful for a model that's so slow.
Unsurprisingly, Supreme Aptimus Zaal gets to use Ancestral Guardians in his theme force as well, gaining an increased FA, as well as Advance Deployment, in addition to all the benefits of running a Guardian with this warlock anyway.
Whether you take an Ancestral Guardian or not is really going to depend on the rest of your army. If you're running lots of cheaper infantry (such as Praetorian Swordsmen), you'll probably find him to be very helpful. Obviously, he'll be useful if you're running Zaal. With other forces, however, you'll probably have better uses for your points. 3 points isn't much, but when you could spend them to upgrade a unit of Cataphracts, or even change some of your beasts for more expensive ones, you'll probably leave the Guardian at home.