Necromancer can farm souls on wisps

[jammer312]

If you kill wisp (at least with chill of the tomb) it'll sometimes refund soul to you. And chill of the tomb guarantees that it'll be resummoned as wisp.

[jammer312]

P.S. Judging from how rare it happens compared to early game I suspect it refunds soul if minion shouldn't spawn a wisp (25% of that as aura passive says) but does so because of chill of the tomb