Board Thread:Warhammer 40k General Discussion/@comment-25618639-20150426193759/@comment-6078851-20150429140929

the only folks i could imagine building the outsider's dyson sphere would be the old ones. if no-one knows it's location, age or... well, anything about it, then i think it's safe to say that it wasn't built by any species still alive :)  and though there isn't really much known of the old ones they were probably a type 3 civilisation.

i left out mention of the craftworld eldar because i'm not sure where their civilisation would fit in this scale: they're definitely very advanced, self contained civilisations, probably on a par with a type 2, but as they're nomadic they can't harness the majority of a sun's power with the construction of dyson spheres (although they probably have the technological capability).

i'm just away to check up on 'o'neill cylinders' as i don't know what they are, although the first thing that comes to mind from the name is a babylon 5-ish construct, which craftworlds are indeed quite similar to... with the notable exception that craftworlds are way better :)