There are no direct rules to represent them, but you can do a decent job.

  1. For your HQ, run Huron and remodel him as some cool Alpha Legion captain. He has a warlord trait that grants d3 infiltrators so you get that.
  2. Have cultists. Using a dark apostle with them is sub-optimal, but fun.
  3. Play several small squads of vanilla marines or chosen. Equip them with melta bombs/melta guns and have them infiltrate/outflank for that just as planned moment.
  4. Super alternative, use the rules for Ravenguard in the Vanilla Space Marines codex. Alpha legion are not particularly daemonic/corrupt so it’s not that weird to use Codex SM to represent them and Ravenguard get scout/outflank on all of their non-bulky infantry. Only downside is you have to use scouts instead of cultists (not a bad trade). If you do this, make your own scout models though. The cultist box isn’t bad, but there are tens of 3rd part models that might do it better. The tempestus scions kit is also pretty great.

Cheers

 

Noob question regarding Alpha Legion from Warhammer40k

Plus one for raven guard chapter tactics, then create your own unique models