My Relictor's are a cohesive looking bunch through 2 things (maybe 3). They follow the vanilla codex, so no gizmos there like Deathwing or Ravenwing, but the things making them look the part are:-
1. Painted Minis. They are all coloured similar with dashes of an accent colour to tie them in. Look at the following mini's. They are clearly all Relictors, but notice the slight accent bone colour on all three's weapons to tie them in, as well as the obvious Chapter Markings.
2. Based. They are all based and weathered the same. The basing article i wrote at the beginning of this blog is still the recipe i use today giving a consistent theme throughout. Here are some earlier mini's and the latest - different paint techniques and standards but bases the same.And vehicles, same Calthan Brown coloured stipple as the bases. And, often overlooked is the edging on the bases, mine echoes the actual earth colour but the whole army has the same edging.
3. I've just thought of a third theme... mine have Wargear or slight conversions throughout to unify - mine signify the dubious links with Relictors using Chaos for fighting Chaos, hence you will see Chaos bits throughout, spikes on Dred, Chaos Smoke Launchers and Chain Links on Tanks, Fallen Backpacks on Sternguard, ample skulls and whotnot...