According to Wikipedia, Heilung is classified as "Experimental folk", while some other use the term "Neo Folk". They describe their music as "amplified history from early medieval northern Europe".
None of those genres are really related to metal.
However, Heilung seems to be very close to the metal world:
- their label is Season of Mist, who is usually focused on metal
- their logo style is quite close to black metal bands style
- they do performances in metal festivals such as Midgardsblot Metalfestival, or Hellfest
- This is only my personal experience, but I observe that those who listen Heilung also use to listen metal