I love the Defiler model I am thinking of converting a defiler into a necron war machine (since I hate the pyramids of death) my idea is this

legs and body of defiler to give it that spider look
Replace head with that of a tomb spyder and add necron like carapace to back of defiler to give it a spider like abdomen
Replace delfier weapons with a heavy Guass cannon and a a gauss cannon

Havent really thought about rules but was going to base it on deflier in terms of survivabilty but with necron weapons and living metal rather than well be back