4

Life Leech

Diablo gains 1 Soul and heals for 2% of his maximum Health when Basic Attacking an enemy Hero. While at 100 Souls, increase this healing to 4% of his maximum Health.

Not the safest soul generator, but it synergies with the rest of the build too well to pass this up. The self-healing plus 100% AA speed after lvl 13 is really good sustain (about 400 healing per second at lvl 20).