So, the idea is that Warrior and only Warrior would get a new kind of armor. This armor would be blue and have spikes sticking of it and would either appear above the armor next the portrait or on the other side of the portrait. Basically, the spiked armor works the same as normal armor except that when attacking the hero, you take damage equal to the amount of spiked armor. Spiked armor does decrease, though and therefore deals more damage the more you have. You lose spiked armor before normal armor. Here is an example card using the mechanic:
Damn... Art is from the super amazing 6th edition Hordes of Chaos army book cover (Warhammer Fantasy). So many good memories... I still have my copy of it!!! :D It is kinda like a treasure to me... ;P
About the card: I like the concept in general, but the execution you proposed feels a bit too clunky for Hearthstone.
This would be my version: "Gain 10 armor. For the rest of the game, during your opponent's turn, your hero's Attack is equal to your armor".
Don't know (neither I care yet) if it is balanced or not for 5 mana, although I'm pretty sure it should cost much more than that. Considere this only a suggestion, my friend, and Happy New Year in advance!!! ;)