There are totally 12 combination artifacts, did you even collect all of it? I have only collect about 5 according to the guide. Where I find Titan's thunder is the best in my pocket.
I don't play Heroes that much, I just play it for fun, online, with my friends. During the games I've only collected 2 or maybe 3 combination artifacts (The Legion, The one with the Vampire/necromancer artifacts)