He will gain 10 hit points no matter what the level of the ability is? In that case, just follow these steps:
1. Create a new ability in the Object Editor, using a passive ability as base (the ability must have a buff, I would recommend "Devotion Aura", I'll use that as an example now) and name it whatever you wish, I'll just call it "Devotion2".
2. Set all data values to zero so that the ability itself won't do anything (we will use triggers for the ability's effects instead) and modify the field called "Targets Allowed" so the ability only effects "Self".
3. Create a custom buff and call it whatever you wish, I'll call it "DevotionBuff". Then go back to the new ability that you made and modify the field called "Buffs" and replace the current buff with your custom buff.
4. Now open the Trigger Editor and create a new trigger.
5. For the event, make a "Unit - Generic Unit Event" and modify it so it says "A unit Is attacked".
6. As for conditions, make a "Boolean Comparison" and change it into "Unit Has Specific Buff". Then modify the fields so that it says "Attacked unit has buff DevotionBuff".
7. Now for the actions. Make a "Unit - Set Life (To Value)" and modify the field that says "Triggering unit" into "Attacked unit". Then click the other field and change it into "Arithmetic" (that option can be found at the top of the list that initially says "Unit - Property".
8. Now you will have two fields that says "1.00". Click the first one and change it into "Unit - Property". Then modify the field that says "Triggering unit" into "Attacked unit".
9. Now click the second field that says "1.00" and change the value into "10.00". If you've done it all right, the action should now say "Unit - Set life of (Attacked unit) to ((Life of (Attacked unit)) + (10.00)".
10. And all that is left is adding the ability to your hero.
I hope this wasn't too confusing, good luck. Also I haven't tested this myself, but I think it should work anyway.