It's hard to say "this is a new unit coming to Forge World," because lore-wise, this is actually one of the oldest units around. They're the Destroyer Squads and you'll soon be able to add these radiation-spewing soldiers to your 40k forces when they make it to Pre-order over on Forge World.
From the website:
They’re back – the bane of landscape gardeners everywhere, shunned by their brothers for their use of proscribed relic weapons, coated with radiation and chemical scorch marks that no amount of lapping powder will get out – it’s the Destroyer Squads, and they’re coming to Forge World soon.
The most brutal squads the Adeptus Astartes bring into battle, these walking war crimes date back to the Unification Wars, and perhaps even the Age of Strife.
Initially deployed during the tail end of the Dark Age of Technology, radiation weapons, phosphex bombs, and alchem rounds were all employed to scour life from worlds across the galaxy.