When I ran regular Friday night/ Sunday afternoon tabletop role-play games I had an NPC wizard, called Blaise, who was centuries old and, while physically in good shape (all things considered) was not so good mentally. He knew spells like Talk To Wall ... but could never remember the bit that let them talk to him. He knew Earthwalk which allowed him to walk on solid ground as if it were, well, solid ground. A tendency to misremember exact wording of spells lead to more than one sentry who was supposed to slump into a snoring, woolly-headed stupor running around baah-ing and bleating and actually woolly!
In the heat of battle, however, he became very sharp. Whenever the party were in deep trouble through no real fault of their own he would bumble in, exclaim things like,"Oh! I say! This is jolly exciting!" before blasting fireballs or lightning bolts at the enemy while healing party members and even dragging the apparently dead back from Death's door.
While the party recovered he would offer to organise food or healing supplies or firewood or whatever. Once the party were out of sight and things were calm, he'd forget all about them and wander off.
If I ever get my tabletop sci-fi campaign back up and running he will be back as a sanctioned psyker of The Imperium Of The Sentient Mind. A useful and, if my old players are to be believed, amusing Deus Ex Machina.
I think your Necron sounds every bit as good an idea. Go for it.