Ranald's Bones is the name given to the item-collecting minigame that happens at the end of any successful mission. It consists of 7 dice that are thrown to determine the rarity and type of the item obtained. The more dice score a "hit", the better the rarity of the loot. The quality of these dice can be improved to score "hits" more easily, as described below (paragraph extracted from the Guide to surviving the Vermintide: For Beginners):
By bringing tomes and grimoires to the end of the level and reaching Ranald's Bones with them, you'll replace one of your regular die with a different, better die. Each die will always either result in a Hit or Miss. Better die have a higher chance of getting a Hit and that's what you want, cause for every Hit you get, you go up in the line of items. So if all 7 dice gets Hits, you'll get the item right at the top of the line.
- Bone Dice are the default dice you roll, you start out with 7 of these, and they get replaced with the better ones you get in the game. They have a 2 in 6 chance of getting a Hit.
- Cursed Dice can be found throughout the game level, and its appearance is completely randomized. These have a 3 in 6 chance of getting a Hit. So check those chests, because you never know when that shiny big block called "Loot Dice" will appear. They can also be dropped by Loot Skaven. These aren't mentioned in this guide, but they are rats who don't try to hit you. Oh, and they carry a large sack over their shoulders. It'll be obvious who these are.
- Tomes give you a Chaos Die, a nifty little golden die that sorta looks like clockwork. This baby has a 4 in 6 chance in getting a Hit.
- Grimoires give you a Skaven Die, really obviously green one. It's a 6 in 6 chance in getting a Hit. Yep, you heard me. Seems like it's worth it now, eh? Don't worry about not seeing it light up during the roll, cause it won't. It's already a 100% chance of a Hit anyway.