The Soul of the Druid

This quest allows you to get the Druid soul for the Cleric. Only problem is, the quest details in the game aren't all that great making it very confusing to complete.

You'll first pickup the quest from Mahal Dewpetal in Sanctum and you are to get a Plane Shard. The Plane Shard is easily acquired by fighting in a Rift and getting loot from it. The Plane Shard will be in the loot bag. From there it gets confusing but is actually quite easy.

Once you have the Plane Shard you'll have a Soul Net in your bag. The item should be on your quest tracker so that you can click it to use. Simply use it at any open rift and it will summon the person you're supposed to fight. I recommend standing away from everyone if at all possible as the enemy you summon is transparent making them a bit difficult to see sometimes.

One you kill them, the quest will update and tell you to go back to Mahal Dewpetal.

This should be the same for the other soul quests as well, you'll just have to fight different people.