Item says: “While at max wraiths, the Wraithlord casts Necrotic Beams and regularly consumes all your non-wraith minions to empower itself, gaining Temporary Maximum Health equal to 10% of their maximum health and +10 spell damage for 10 seconds.”
In fact, he cast regardless if there are any wraiths summoned. When there are enemies nearby, he follows that rule, summons the maximum wraiths and then consumes other minions. But without enemies, he doesn’t summon wraiths and just consumes minions, even if there are no wraiths at all. This happens even if I don’t specialize into wraiths, so that’s not some sill node interaction.