Legendary Cruelty?

[PvK]

There's also the one that gives increased Magic Resistance, fairly common for me.

Toughness is pretty common for me. My bet is it is mainly just random, though it seems to me that when multiple units get heroic at the same time (like when you send three commanders into the same battle on turn 2), they often all get the same ability as each other.

[Kristoffer O]

I'm not aware of any correlation. Abilities should be random, with a couple being rare, unless JK has changed something without my knowledge (which might happen).

[JimMorrison]

People will always see Elvis, they don't need JK's help with that.