Andy Smillie
Sins of the Father

As Azkaellon and Amit duel in the ritual Tempest of Angels, the two favoured sons of Sanguinius learn something about their virtues and their weaknesses, of both themselves and each other.
The Horus Heresy
short

Also in:
Imperium Secundus
Virtues of the Sons/Sins of the Father
Eye of Terra