In the Lords of Olympus setting, people can die. NPCs can die, players can die too. And there's technically no resurrection spell.
However, in one sense dealing with mortality is something that may have more options in LoO than in most other games; the dead are potentially everywhere.
First, there's the multiverse. Your favorite manservant died in battle? Your girlfriend was murdered by one of Apollo's agents? Well shit, you can go out and find a clone in a world almost like the one they were from. This might even get used against you: an enemy might lead you to think your supposedly-dead girlfriend is still alive; and of course as an Olympian you know that she could just be a copy from some other realm... but can you be sure?
Because there's another detail too: everyone who ever dies in Lords of Olympus LITERALLY goes to the Underworld. And the Underworld is an actual place. For an Olympian, its kind of like if in our world everyone you ever gave a fuck about who "died" just ended up in Detroit. You can visit Detroit (though its not usually very nice), you can search around to try to find people in Detroit, if you find them, you might even be able to get them out of Detroit alive (assuming you can figure out a way for them to get a body back)! Of course, Hades does not care for the idea of people coming into the Underworld intent on taking away HIS inhabitants; he's a bit of a fascist that way.. or it could be something about "cosmic balance", its hard to say. In either case, if that manservant tied on your armor in just the perfect way, so much so that its really worth the costs, you're going to have to either cut a deal with Hades, trick Hades or oppose Hades if you're going to try to get Manny Armorties back up to the land of the living.
And if you can't get there or do those things, you can still also just call "Detroit"; have you got Advanced Scrying? Then you can try to communicate with the spirit of any dead mortal you ever knew from the safety of your own home Realm. If it was Manny's advice you valued more than his armor-tying skills, then you can still consult with him on the problems and tribulations you now face after his unfortunate demise (assuming he's not pissed off about the 23 spearheads in the back he got while helping you escape to safety with nary a scratch; in LoO you can go find the dead without much challenge but that doesn't mean they'll always want to talk to you!).
Ironically, there's one exception in the whole wide multiverse to all this: the Gods. Those who have Immortality do not live on as spirits in the underworld after death. And you may find some reasonable facsimile of them out in the multiverse, but its just not the same. There's only one Hermes, only one Aphrodite, and if they (or any PC who has Immortality) should die, their like will never be seen again. The Gods alone are truly unique.
Note that a clever player might think this through and decide NOT to get Immortality, so that if he dies in battle he could just be rescued from the Underworld, and while strictly speaking the GM should not just outright deny this possibility he should consider the following points: first, Hades will particularly want to keep as his thralls those who were of Divine Blood. When the day comes that he must march out his armies to fight some great foe, it is these heroes in particular which give him an edge... maybe even an edge over Olympus! Second, the PC will be as likely to have people, personal enemies or family enemies, who will want to NOT see him back as he will those who want to save him. Third, unless the PC made very clever preparations indeed, he is likely to be either lost in Asphodel or partying in Elysium; in the former case he will have no memory whatsoever of his former life; in the latter, he will be experiencing such pleasure and bliss he will have been 'enchanted' by it not to wish to ever leave. Though this would pass after leaving the underworld, the PC needs to not only have friends willing to save him, but friends willing to drag him kicking and screaming back to life against his own will.
And of course, the final salient point: this is Lords of Olympus. If you've really got a game where all the players are totally peachy-keen on the idea of teaming up to save a fellow player's dead PC at risk to their own PCs' lives, freedoms or power, then you're probably running it wrong.
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