When an ally falls in battle you have the option to save them for half your life or sacrifice them for a super attack and then they watch as a ghost and offer slight help like boosting players and weakening monsters

The offerings(spells) and black rites(extremely powerful attacks that penalize your player afterwards) have to be chosen wisely as well

Other players can choose to sacrifice you in order to do devastating attacks, even against your will - but you will be rewarded for it

See also the quite large collection of lore that is unlocked throughout your advancement, but it will not only provide very interesting stories, it also gives hints to bosses weaknesses and info on their deadlier attacks

Sacrificed players can then act as spirits, who can see enemy lifebars, bolster attack and defense, and shout out phrases from a list of pre-picked options

You will need to be able to see the ways a monster telegraphs its attacks before they happen and you will need a firm grasp on the range and timing of your own attacks

Even the really powerful attacks have a cost so it encourages you to employ balance

at least their attacks don't hurt you

Sacrifice boosts your magic/attack while Save grants you additional recovery and defense