I think why Parvell was making that expression is because not only was Sion's stigmata revealed when he had been trying to hide it, but he was also consumed by the flames. Parvell is probably blaming himself because things didn't go according to his plan which resulted in this situation. And he also didn't let Sion know about his plans, as usual.
The reason why the reveal will have a negative impact is because:
1) The future will no longer follow the past life timeline as closely as it was. This changes everything so neither of them will be able to make as accurate predictions anymore to protect each-other, and others
2) Duke Mayee was trying to find out who had Zeon's stigmata. Parvell already killed the duke's spies and sorcerers who were looking for the stigmata. The only reason the duke didn't want to kill Parvell yet was because it would awaken Zeon's stigmata with the death of the god of judgement. So now Duke Mayee knows who has the stigmata he will probably be trying to kill Parvell
3) Sienna is known as the ragged king. Some people might think he stole Zeon's stigmata from the rightful owner, and some people might try to spread this type of rumour which will cause a massive controversy
well mc will be more prone to danger and this means parvel’s death is near
Also with Siena having the legitimate right to the throne trough the stigma, the Mayes will do everything on their power to make him a puppet of theirs.
Wait, why did parvell wore that expression, how would stigmata revealing itself negatively impact sion
Damn, i’ll have to re read this from the start