Therefore, in your example (Death Knight vs. Hydra), assuming primary hero skills are 0, there are no attack or defense secondary skills, and the heroes have no attack or defense specialization, and the battle takes place on neutral territory, the following will happen:
If the Death Knight's damage is determined to be 20 as a result of a random number generation (RND), then if luck triggers, it will deal 40 damage, and if double damage also triggers, it will deal 60 damage.