You get the quest from the Pious Man in the Waterfall. He has a quest that asks you to find the Warrior's Heart.
To start, talk to the pious man.
When you take the Warrior's Heart quest it gives the Freeman buff which allows a leader to quest partner with you.
Taking the second part of the Warrior's Heart quest removes your Freeman buff and makes you the leader. You will get a quest to find a particular class and partner up with them.
Leader finds the class they are asked to find. Form a party with the leader as lead. Have the partner take only the first part of the Warrior Heart quest so they get the partner Freeman buff.
If they take the second part of the quest, their Freeman buff will be gone. And, when the leader tries to take the "found a partner" part of the quest, they will get a message saying that they don't have the correct buff and they will have to find a new partner. The partner can take their quest back a step to get the buff back by using the Reversion crystal sold by the Pious Man for 88sg. The crystal can also be used by the lead to reset their quest and try for another class if they can't find someone from the class that they got (such as, couldn't find a mage, so reset and got an assassin!)
The leader talks to the Pious man again and is asked if they found the class they were looking for.
Leader clicks the quest and each member will get an item in their bag - Warrior's Guidance. Each member right clicks the item to get a quest (which tends to be the same quest for both, but can be a different quest for each member).
Go complete the quest(s). There is a timer on this quest, but I have gotten it to complete for
both members even when the timer runs out - not sure what the time is
actually for...
Each member will get a Warrior's Belief once completing their quest.
Each member will get a Warrior's Belief once completing their quest.
At this point, each member's quests will go green and they will be able
to talk to the Pious Man to get their box (it is automatically placed in
your bag by the npc when you hand in your quest).
The boxes contain a variety of things. Some things I have gotten so far:
Spirit whistles x 10
soul diamonds x5
mastery pages x8
lv2 gem box
ancient warrior souls x8
resistance pages x10
Here's a list of all the possible rewards and the rate at which they drop:
Kill a frenzied demon
Pick a stone <-- easiest one
Kill a series of 3 different mobs, 20 mobs each: spiderladies, corrupted priests, rats
Note: We had some trouble with the frenzied demon quest. It doesn't like to clear if the person
doesn't kill it themselves. My priest had to be the only one to hit the demon in order to get credit. My mage just had to have the most hits on it to get credit. Each of them had aggro when the demon died (even the time that someone helped the priest kill she had aggro at the end, but her kill didn't count - it only cleared when she was the only one to hit the demon).
Also, adding another person to your party can make the quest fail - as Benniee mentioned in the comments. I added someone to help kill - the kills from the 3rd person did count and it didn't affect the quest, so this also seems hit and miss; BUT it tends to make the leader's quest fail. I had it fail on me twice and it was always the leader, and Benniee's failure was of the leader's quest too. I even tried removing the third person from the party before handing in the quest to the Pious Man, but it still failed. (Perhaps if the third person were removed before the kill quest was complete?)