The bosses are not related to the demdrakes but they are related to the rift beasts. They usually are large and can spawn rift gates. But they are merely temporary.
Boss 1: The Void King
The void king is the worst thing of all. Its huge and it can open permanent portals that can summon other bosses. Also anything else but your true form cant hurt it. So make sure you have enough life energy to beat him otherwise its going to be really hard.