Under what conditions does Malthael drop legendary pieces or plans?

Solution 1:

The fear is random, some people need to farm him over and over to get 1 When you first kill him you get a guaranteed legendary, thus the boots. Reset your quests and you'll get another. The plan all my friends got on their first kill as well so not sure.

Solution 2:

As of patch 2.0.5. Malthael will always drop the plan at level 60+, as long as the player does not already have the plan.

The material remains a random drop from Malthael.

Malthael will now always drop the plan for Reapers' Wraps when killed at level 60 or higher if the player does not have the recipe, instead of only on the first kill