The plot card "Ascension - The Harbinger Approaches" says:

All four Temples must be in play to play this card. Place one Comet token on a Temple of your choice when this card is played and at the start of each week thereafter. All heroes have their maximum wounds and fatigue reduced by one for each Comet token on a Temple within three trails of their current location. In addition, if you ever end your turn with nine Comet tokens in play, you win the campaign. The heroes may enter your Keep while this card is in play.

What happens if the OL has 8 comets in play and a hero with 8 life moves into the influence area of all 8 comets?

Maximum wounds reduced to zero, hero out of the game?

uhm... i think probably hte heroes will remain with minimun 1 wound. ithink even with 0 wounds they still play and dead at first strike that pass their armor. because a group with 3 or less heroes it's usless to play, the OL have simply win, in the other way they still have a chanse dressing heavy armor.

but it's only my opinion i was never be so good to put all that comet tokens

AFAIK there is no official ruling. A few options have been presented in past threads about this:

1) The hero dies at the start of every encounter and whenever he enters a dungeon. The OL gets the conquest. If he leaves town in a dungeon, he'll die instantly yet again (granting conquest as usual).

2) The hero dies and stays dead (at least while the party is within range of all 8 comets). You've got 3 heroes now, better run away or give up.

3) The hero sticks around with 1 wound.

In any case, the heroes shouldn't have let it get that far in the first place, and will need to rush off to a secret training area to get 4 extra wounds if they want to stand a chance.