When Rai uses his special attack on Urokai and Zarga, he turns into a giant Phoenix-like creature. We've seen similar transformations when each of the Clan Leaders used the special attacks of their Soul Weapons (Seira has a Grim Reaper, Zarga has a Dragon, etc.). Since Rai has no Soul Weapon - with Gajutel telling the Lord that Rai is pretty much a Soul Weapon due to his power - the answer to his powers might lie in that very trait. Since the Soul Weapons materialize a form when using a special attack, that might be the truest essence of the past Clan Leaders' souls; Seira's Grim Reaper could be a transformed version of the original Loyard Clan similar to Rai's Phoenix form.
Next, the transformations would get stronger as each successive Clan Leader went into Eternal Sleep and passed down their now-soul infused weapon to their heir (next Clan Leader). Now, in terms of Rai, since he has no Soul Weapon, a question comes up - since his power is bound to his blood and body rather than a material form, does that mean that his power was always as great as it currently is (minus the fact that his body is weak)? If each Clan Leader gets a stronger weapon than the previous generation, and Rai remains the strongest regardless, then does that mean his strength was God-tier from the beginning - albeit a glass cannon?
I believe the answer lies in his transformation. If his power is that of the Phoenix, then it's possible that Rai himself is reincarnated constantly, thereby allowing his power to increase during each generation. It has already been stated that:
- The name Noblesse was given to only ONE person
- Rai is THAT Noblesse - The Strongest Noble
- Rai's power is in his blood
- Rai has a Temple similar to the one where the past Lords go to Eternal Sleep
- Rai uses that Temple to 'hibernate' for extended periods of time
So, instead of 'dying', Rai is just reborn each time. The cycle of death and rebirth is intertwined with Rai's transformation into a Phoenix; as noted by Frankenstein, the Soul Weapon is linked to the trait of clan, thus Seira's Clan of Death is linked to the Grim Reaper as Rai's transformation into a Phoenix would be to the cycle of reincarnation. That would also explain why a Noble like Gejutel, who looks much older than Rai, is in fact younger - it's unimaginable that a Clan Leader who was born before the Noblesse wouldn't know when Rai was born.
This trait of rebirth could also be a method to preserve the function of the Noblesse; as he is the eternal judge of his species, it would make sense that the same being would remain as that judge for all eternity - that is until the Nobles have changed enough, as per the wishes of the former Lord, thereby allowing Rai the chance to finish his duty once and for all.
Furthermore, the traits associated with the (East-Asian) Phoenix is illustrative of Rai:
- Virtue of Benevolence, Charity, and Humanity
- Honesty and Uprightness (incl. Altruism, Consideration for Others)
- Knowledge
- Faithfulness and Integrity
- Correct Behaviour, Propriety, Good Manners, Politeness, Ceremony, Worship (aka ELEGANCE)
Now, with this, there's the question of why didn't Rai just reincarnate himself after fighting the traitors 820 years ago. Well it could be that his reincarnation process could've been hindered by the traitors. Furthermore, there could be a significant enough gap between the death and reincarnation process which could've been risky if the traitors decided to wage full war against all the other clans and Lord. Moreover, it could have to do with the need to hibernate on a night with the full moon; if he was attacked on a different night, he wouldn't have access to his coffin, etc. Also given the nature of his wounds, I doubt he even made it back to his mansion.
The last consideration is why the traitors were so surprised to see him alive. If his power was reincarnation, then they would've known he'd be back eventually. However, that's simply assuming that they knew his all of his powers. Since he was feared by his own kind, it makes sense that they were unaware of the extent of his abilities thereby allowing leeway for the secrets behind his powers and link to the Phoenix.