From Halopedia, the Halo wiki
The rifle has design similarities to the Zubo-pattern beam rifle and M'tara-pattern focus rifle sniper weapons employed by the Covenant, and the larger weaponry employed aboard the similar Bloodfuel Locust. However, it is mainly used for close-range combat. The weapon has two identical spikes on the front of the it, it also shoots orange projectiles despite having a green glow in the interior.
Infantry deployed with these weapons wear tank-like backpacks, though their function is unknown.
The bloodfuel rifle only appears in the Blitz multiplayer mode of Halo Wars 2. The weapon is wielded by the Bloodfuel Grunts & Brutes units with the unique capability to siphon enemies' health, exclusive to the Decimus leader.
In a Twitter thread discussing the canonicity of the Extractor Marines and their canon status with the context of Halo, writer Jeff Easterling commented that it was "odd territory getting into draining "life force" no? Lol.", and that "Doesn't mean the weapons won't be canon necessarily, but canonical functionality could change :)", reaffirming that the weapons do not in-canon ""steal their health and give it to you"". The weapons are canon, but their function as depicted within the gameplay of Halo Wars 2 is not.
List of appearances