However, whereas Wraithguards and Wraithblades house Aspect Warrior souls and Wraithlords house Exarch souls, who are then shepherded by a Spiritseer, the Wraithseer cuts out the middleman and puts in a Spiritseer's/Warlock's soul stone as the pilot, allowing them to exert greater control over his wraithbone shell and the shells of others. In a way, they resemble the Blood Angels' Librarian Dreadnoughts, except they're not quite as ridiculous a premise. However, they're just as grand a rarity, as the secrets behind making these psychic machines is lost to the general populace. Also, despite the badass practicality, these titans are used more as a last-ditch resort, more so than using Wraith-constructs normally entail.
On the field, it is a legendary thing to field if your opponent lets you field Forge World. It has both incredible support for other Wraith-units and for combat a Wraithseer is always equipped with a Ghostspear and Wraithshield, and may also arm themselves with a Bright Lance, Scatter Laser, Eldar Missile Launcher, Starcannon or D-Cannon.
Their psyker abilities are also quite potent, allowing them to assist friend and hamper foe. Wraithseers have been known to cloud their enemies' minds with the terror of death, enliven the spirit stones of ghost warriors around them to inspire fresh vigour, and alter fate to protect themselves and their charges.
On the Tabletop
8th edition Wraithseers are the single most durable HQ choice any Craftworld army can take, and are arguably the hardest hitting as well. This lethal combination of durable and powerful makes them quite the DISTRACTION CARNIFEX, as anything short of a Titan is easy prey to the high damage weapons the kitted out Wraithseer brings to bear. As mentioned above, it has great synergy in a Wraith-heavy list, and can give accompanying Wraithguard/blades or Wraithlords an extra boost in mobility or endurance as the situation demands.
9th edition Wraithseers got quite a tweaking. Though they are sadly no longer an HQ unit choice and lost the Character keyword, they did get a few buffs that make up for the lost Warlord Trait/Relic slots. The first major change is that they lost three wounds, dropping down to 9 total. At first glance, you'd assume that's a terrible change! But then you realize that dropping below that 10 wound threshold now means that the Wraithseer no longer has a degrading statline hindering it's performance on the battlefield as it gets worn down. Additionally, it treats all AP-1 weapons as AP0, making a lot of the cheaper, standard-issue weaponry a bit less likely to chip away at it than before. Lastly, the best deal is that the Wraithseer is now a full blown Psyker, dropping the three unique (and limited) powers it used to know in exchange for Smite and one set of Runes of Battle. This allows the Wraithseer to support all of your Craftworld infantry, not just the dead ones, and it can now become particularly threatening thanks to the myriad of debuffs it can now place on enemy units.
Offensively, the Wraithseer comes stock with a Ghostspear (S10 AP-4 Dd3+3 Damage) which lets it pry open the hull of any vehicle it comes across and wound most standard infantry on a 2+ while erasing pretty much all GEQ and MEQ saves. This can be complemented by one of several heavy weapons its Wraithlord cousins usually carry, but you'll almost exclusively want it to carry a D-Cannon. When equipped with a D-Cannon, Wraithseers become fearsome anti-vehicle/monster combatants more than capable of downing a Leman Russ tank in a single turn, though they'll struggle at wading through thick blobs of GEQ hordes simply due to lacking the volume of shots/swings to carve through them all.
Defensively, while the Wraithseer is extraordinarily sturdy at T8 and a 3+/5++ save, it does have a few shortcomings. As a massive model, chasing cover or LoS breaking terrain will prove difficult in the best of times, if not just blatantly impossible in the worst. Additionally, as it is a monster, the only psyker support it can benefit from directly is that from a Farseer. Las-cannon and Multi-Melta grade weaponry can and will ruin a Wraithseer's day. Fortunately, it will remain combat effective regardless of how much non-lethal punishment it takes these days, as the damage table of yesteryear has been lifted from it's shoulders and it'll be able to shrug off lower strength/AP fire off with greater ease than before.
As a Psyker, the Wraithseer is now substantially more flexible in it's offensive/supportive range, only limited by a rather unfortunate single cast/deny per turn. Smite, as the golden standard, has finally been made available to it and, by proxy, the Runes of Fortune as well. The Wraithseer's non-Smite slot power is taken from the Runes of Battle these days, which grants the Wraithseer considerable direct infantry support or fantastic indirect support for the entire army through the debuffing capabilities afforded to it. Compared to the substantially cheaper Warlocks/Spiritseers and the fact the Wraithseer now competes with units like the Dark Reapers or slightly more offensively oriented Wraithlord, it is a bit hard to justify using it as a supportive unit for much of your army.
Counters: If you're facing an enemy Wraithseer, there are a few fairly standard ways to deal with it. Standard anti-vehicle/monster weaponry can drop one in as few as two or three hits, with S9 grade (Lascannon equivalents) and above being particularly proficient at punching through their toughness. In general, AP-2 or AP-3 is plenty, as it will surpass the AP-1 negation the Wraithseer has. If killing it outright seems a tall order, you can simply bog it down with bodies. Though it hits damn hard, the Wraithseer only has four attacks a turn and can't fire its D-Cannon in combat (as it's a blast weapon). Swamping it with relatively disposable bodies will prevent it from being able to fire it's main gun or from charging into something of yours considerably more valuable to your war plan.