The effect can stack up to a maximum of 50%.
Feat Effect 2: Dragons' Assembly (once a year). Upon use, summon all Dragons from the current world to a location you designate; all Dragons of the current world will receive your summons, and the targets will decide whether to accept your call based on their distance, their relationship value with you, as well as the current environment.
Feat Effect 3: Domination of the Dragon Race. You can dominate a Dragon to serve you, and the target cannot refuse any of your orders that go against their will. The probability of successfully dominating a hostile Dragon is reduced by 50%, while the probability of successfully dominating a Dragon in a state of fear is increased by 100%.
Heroic Feat 3—
The Immortal (Mythic Feat). You have survived the full-force attack of the high-tier Divine Artifact, the Dragon Soul, learning skills to withstand attacks of such overwhelming power; your physique has thus been strengthened.