If you don't intend to reduce the upgrade later, you can use an Attribute Bonus ability that doesn't show an icon, and an upgrade based off Skeletal Mastery that increases the level of the Attribute Bonus ability.
I would not advice to use the attribute bonus ability in this case since it is just unneeded overhead, unless you want to increase the stats of multiple heroes owned by the same player at once.
If this is for just a single hero you should better not use any ability at all.
This is because you can manipulate the attributes of heroes directly with triggers, setting, increasing and subtracting attributes at will. No need to waste additional memory for ability levels and effects.
If you trigger an attribute increase, you can't detect the amount of bonus attribute a unit has, as it changes the base value instead of adding on to it. Nor can the player tell how much his attributes have increased - he only knows that he has higher attributes.
I'm on a page about incorrect corrections, and spent the better part of like two hours trying to get someone to understand that -5^2 = -25, not 25, and then that post had comments get reposted because that group is self sustaining, and that person was in turn trying to explain what I just explained to them. And I'm taking that as a victory
I will be AFK for a couple of days you guys hold down the fort while I am gone. I will be checking in on my phone but that will severly limit me. Be back saturday!