Alright, so here's the deal with the Necron Overlord.
- Can pick between either a staff or a warscythe. The staff will make him do ranged attacks, the warscythe will make him do melee. He can only fire while stationary and targeting is kinda buggy (as are melee attacks). The warscythe can't do ranged attacks but the staff can do melee. Most/all of his attacks have high armour negation.
- Every weapon gives a few buffs and/or abilities. Choice of weapon is a major factor in what kind of build you'll have.
- The Necron Overlord generally has very high armour and thus damage reduction. A very nuker-oriented build can become quite fragile but even then usually has room for defense. A tank-oriented build (particularly melee ones) can be extremely durable, with anywhere from 60 to 75 percent damage reduction (against non armour ignoring attacks), high health regen, abilities for even further health regen, a Tau Commander-style restore-health-instead-of-death thing, and even an ability that makes the Necron Overlord automatically self-resurrect whenever he dies (takes a bit longer than a teammate resurrecting and if the entire team is dead at any point the game will end anyways even if it's just for an instant before your self-resurrect finishes).
- Has some powerful nuking options. At level 20 he gets to lay down an extremely high AoE DoT that will wipe out basically any infantry (including Wraithguard) that enters it. If the team is smart and pulls this can finish off most/all of a wave by itself, and due to the position it takes in loadout, it is very easy to take even if you seldom use it. Also has a less potent but still effective AoE health drain. Energy intensive but great for survival and clearing trash mobs.
- The most powerful builds I've seen tended to combine the Voidbringer scythe with damage boosting armour (forgoing the self-resurrect power though this could be because most players haven't unlocked that armour yet), the +100 armour boost, temporary huge regen boost, general regen and health buff, and just run in and kill any single target really fast, one at a time. Not good at clearing big mobs of enemies but very good at killing the tougher ones and just not dying in general. This build is also immune to knockback, knockdown, and even suppression.
- The Necron Overlord is fairly slow. Moving at a decent speed requires an equipment slot; the teleport ability is very short ranged. This is generally not a problem on Anvil which is small. More of a problem on Colosseum.
- Speaking of Colosseum, going against a scythe Overlord on wave 16 is absolutely brutal. It can one-shot most players. At the same time the Overlord's really powerful AoE snare (which forgoes the earlier mentioned AoE DoT that kills almost everything) can effectively pin and stun the clones and allow a reasonably coordinated/got-its-shit-together team to finish the wave easily.
- There's also some utility stuff like making weaker enemies flee or making an enemy fight for you for a little while; standard shit.
So basically you pick between a somewhat cumbersome ranged damage dealer (with some snaring and nuking options) that plays somewhat like a Farseer, or you play a high damage, regenerating melee supertank that specializes in going after heavy targets and not dying (contrast with the Captain who is good against everything including large groups of trash, and has much better mobility, but is definitely not as tough. I have not seen a Captain in several hours of play today so I haven't seen how well they play together but I imagine a regen-on-strike Captain making the Overlord even more unkillable).
I suppose you could also make a more AoE oriented supertank as there are two warscythes that actually do work pretty well as anti-group, but you would be much less effective against tanks, walkers, etc than a Voidbringer user.
tl;dr can be either a godly tank or an ok nuker.