"AoE damage on death" doesn't show an icon in the command card because of the nature of the spell. The game designers wanted it to be there without it seeming to be considered an ability, per se.
Your best option, in my opinion, is to make some dummy spells based on a passive unit ability, but changed so it doesn't do anything. Make one for each level of your hero AoE death ability. When a hero gains a level of the ability, take away the previous dummy spell and replace it with the next level dummy spell. This will make it seem like it's putting up an icon.
I've used this for the Orb of Corrupt armor reducing and the Tornado's slow aura, and it works snazzy. | 
