From Halopedia, the Halo wiki
Extractor Marines are equipped with a form of backpack and what appear to be modified ARC-920 railguns. These weapons are seemingly capable of "siphoning" the health out of the target of the weapon and healing the marines wielding them.
The Extractor Marines are classed as core infantry, and performs well against infantry, vehicles and aircraft.
While the Extractor Marines and the weapons they wield are canonical designs with the context of Halo, their function as depicted within the gameplay of Halo Wars 2 is not. In a Twitter thread, 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"".
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