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For a 500 points and up witch HQ would prob out the best,in the gaming group there are chaos space marines dark eldar eldar grey knights imperial guard space wolves and Tau each game i have had with my Necrons i get slapped around and never make it across the stupid table lol and i have use the ghost arks to get the warriors a crossed the table but he gets shot once and then done and then i gota make the long walk across the table
Overlord, orb phaeron, Ws, MSS
that 20 man squad can move and shoot with lord attached, has some CC potency, and independant characters will be forced to fear the lord.
I'd recommend either play a 400 point Combat Patrol where you only need 1 troops choice and no HQ, or play 600+ point games so you can actually afford to take a HQ and two troops (plus fancy toys).
But yes, just a basic Overlord with a Warscythe and Res Orb is a good start. Take warriors if you're playing low point games, or a combination of warriors and immortals if you have more points to use.
"Peace, through superior firepower."
The Yukon, keeping your Baals cold 24/7
At 500 points? Eh, just get an Overlord with Warscythe and CCB. Basic configuration and you can use him for a while. As you move up in points, add the non-visual stuff: MSS, 2+ saves, etc. Add other units to flavour.