Necron Lord
The Necron Lord first shows up as a playable unit in Dark Crusade and again later in Soulstorm.
The Necron Lord is easily the most powerful Hero unit in existence in these two games. There is literally no point or way of denying this. When fully upgraded with the right abilities he is an absolute monster on the battlefield in Dark Crusade, and Soulstorm brings a specific couple of upgrades to both him and one of the Necron infantry units (Immortals) that make him more powerful than every Hero and sub-Hero of every other race COMBINED as he is both the absolute master of army control (both his own and the enemy's) and is the only hero that, when fully upgraded, can single-handedly crush any and every Relic unit in one on one combat.
There is one somewhat unique aspect of the Necron Lord compared to other heroes regarding his death and recreation; Unlike all of the other hero units, if the Necron Lord dies and gets remade he doesn't get recreated back at the main base. Like the Necron infantry his corpse stays where he dies, and if he is remade he resurrects right there where he fell. This can be problematic if the enemy parks a large force next to him, as the resurrection animation takes a few seconds to finish before he can teleport away but he can take damage during it, so it may be wise to put friendly forces close to him before you try.
Dark Crusade
The Necron Lord on his own before any major upgrades is fairly decent at melee combat, rather lacking in ranged combat, has a moderate movement speed but a very effective Teleport ability that has long range and a short cooldown. His frightening power comes from a highly adaptable skillset; the Forbidden Archive building allows for the research of up to three main Artifacts out of a set of 8, each of which gives him a unique skill. On top of this he can gain the ability to temporarily transform into the Essence of the Nightbringer, an invincible health-stealing monster that looks like a Grim Reaper and rips anything including Relic Units to shreds in melee combat. The available artifacts are the following:
Resurrection Orb
One of the Necron Lord's most powerful and notorious abilities, the Necron Lord slams a glowing orb into the ground that resurrects ALL dead Necron infantry around him in a decently sized area free of Energy cost up to about 4 support cap above your current max capacity. If only a couple of members of an infantry squad have fallen and that squad reinforced back to full quantity, the resurrected units split off into a brand new squad of their own (they do consume Squad Cap). This can actually surpass the maximum Squad Cap by a limited amount allowing for a very large army to be fielded, and in the Campaign in particular this is even more powerful when done to Honor Guards; Honor Guards do not consume normal Squad Cap which means, theoretically, they can be Squad-Split an infinite number of times to gain an insanely large army. You should always, ALWAYS choose this as your first Artifact.
Nightmare Shroud
Another very useful ability, the Necron Lord lets out a scream that sends every enemy unit within a certain range that isn't a vehicle scattering in all directions for several seconds, including non-vehicle Relic units, and anything capable of having its morale broken has it broken. While the units are running they do not fight back in any way, allowing your forces to attack them unopposed at will for those several key seconds. The disruptive ability of this is undeniable.
Solar Pulse
The Necron Lord causes a large green pulse of energy to flare over a reasonably sized area for several seconds. In this area infiltrated units are revealed, but more importantly the visibility of all enemy units within the field is dropped to almost nothing almost completely negating their ability to attack with ranged fire. This is an excellent way of crippling the attack ability of heavy ranged vehicles that Nightmare Shroud doesn't affect.
Phylactery
The Necron Lord is permanently covered in a swarm of floating nanomachines. This significantly reduces the amount of Ranged damage he takes as well as doubles his health regeneration rate. Without this he can be dropped without too much difficulty from reasonable ranged fire, so it is well worth considering.
Lightning Field
When the Necron Lord engages in melee combat, this passive artifact makes small sparks of lightning fly out of him and cause damage to the enemy. It also builds up a charge that can be released at will to send a strong blast outward to damage infantry.
Phase Shifter
This artifact allows the Necron Lord and all squads close to him to phase out of reality temporarily. They become immune to all attack but also can't fight back for its duration, making this more of a panic-button escape move. Given the Necron Lord's and much Necron Infantry's ability to teleport away from a fight this artifact is a bit redundant.
Veil of Darkness
This artifact creates a permanent cloaking field on and around the Necron Lord over a moderate area. The Necron Lord and every friendly unit, including buildings, inside the area are permanently Infiltrated until the Lord moves out of range or takes heavy damage.
Chronometron
This artifact allows the Necron Lord to change the flow of time around him and his allies within a moderate area around himself. Enemy units move much more slowly but the Necron Lord moves at a normal rate.
Essence of the Nightbringer
The Necron Lord's "ultimate" ability that he can always research independent of the other three Artifacts chosen, this turns the Necron Lord into the Nightbringer for 60 seconds. The Nightbringer form is invincible (although it can be stunned), regenerates health with each attack (its starting % of health is the same % that the Necron Lord had when the ability was used), melee-only and will destroy anything stupid enough to stay next to it, including any Relic unit in existence. He does, however, lose his Teleport ability for the duration as well as any Detection granted from a wargear, so repeatedly stunning him and keeping him occupied with numerous cheap infantry units can "lock" the Nightbringer down.
One interesting (and frightening) note; in the Campaign when going against the home Necron base, you will face one of these close to the bomb site. Have fun with that :).
Soulstorm
In Soulstorm the Necron Lord retains all of his abilities from Dark Crusade, but also gains one new ability that is so ridiculously powerful it literally does make the Necron Lord stronger than every other hero in the game combined.
Essence of the Deceiver
This is a similar ability to Essence of the Nightbringer where the Necron Lord transforms into the Deceiver temporarily, and the two abilities have a shared cooldown. The Deceiver comes with multiple very dangerous skills.
He has a passive ranged attack that doesn't by itself do much damage but has a chance of taking control of the hit enemy squad temporarily, making them fight on your side. He also still has a teleport ability giving him excellent mobility.
Deceive is a more powerful active version of this; all enemy squads in a certain area fall under your control temporarily. Already being downright lethal to the enemy forces, the real kicker to this: Stand-alone Hero units do not fall under your control, but any Hero connected to an infantry squad DOES and you gain access to all of their skills that don't cost Requisition to use. If you manage to grab a hero with strong skills like a Farseer, the enemy's day is going to really, really suck.
Grand Illusion creates a temporary fake Restored Monolith. It has the health of a Restored Monolith (and can be targeted/destroyed as such before the timer runs out), its attacks do no damage but the Particle Whip still does the knockback, allowing for excellent disruption of enemy infantry. The most powerful aspect of all of this ability, however, is that the illusion is a valid teleport target for your infantry. This also combines with another very important upgrade in Soulstorm: Immortals gained the ability to teleport, which they don't have in Dark Crusade. In Crusade you could do a major army jump after a regular Restored Monolith teleported into an area (such as inside a base), but mainly only anti-infantry units and it can take a while to get a Restored Monolith over to a base, even with its teleports. The Restored Monolith's Teleport also stops working if it takes enough damage, and of course you need to capture a Relic first to be able to build it in the first place. Now, in Soulstorm, without needing to capture a Relic first, you can get the Necron Lord to quickly teleport over to and well inside an enemy base, summon the Deceiver, pop out a Grand Illusion and teleport in a massive anti-everything infantry army from anywhere on the map. This very nasty move is known as the Monolith Stomp. Virtually any base that isn't a cloaked Eldar base will be levelled in maybe a minute from this move unless they have their main army right there already to counterattack, which may not help much anyways if the Necron Lord uses Deceive on them. This is brutally effective in the Soulstorm Campaign against most Strongholds; only two are immune to it, the Sisters of Battle stronghold due to the invincibility given by the Living Saint and the Chaos stronghold due to the Warp area that both damages your units and gives rapid regeneration to the Chaos buildings. For everything else, get your Necron Lord next to the key building/unit, pop this off and oh look, you win :).