The quest with the Assassin's Guild was never implemented because Nival didn't add all the promised functionality in patch 3.1.
After completing a certain "secret" quest for the Assassin's Guild (killing a mage on the island), the player would have the ability to hire assassins and saboteurs who could apply various penalties to enemy heroes at the player's choice (with a certain probability of success). But without a trigger at the beginning of the battle, this idea cannot be implemented. So, the Assassin's Guild is currently only for atmosphere...