Artifacts are items gained using Relics. They are powerful items that can change the game, such as increasing the amount of Gold dropped by Titans or making Skills cost less mana. Some artifacts can make the game a lot quicker, whilst others can provide little help.
Once you start to level artifacts, and they start to exceed more than 1,000 levels a click, go to settings and turn on Percentages, so that when you level your artifacts, you can easily level them much quicker with 1%, 5%, 25% and Max options instead of doing 1,000 increments.
Artifacts boost certain stats, and each effect may be better or worse than average depending on your playstyle. So don't just look at the tier rating, also look at the actual effect and how it fits into your strategy.
To get and upgrade artifatcs you need relics, which are gained by using prestige. So the basic idea is that you do several prestige cycles per day to get a lot of relics, which will be a VERY long grind.
When you start playing, there is one very important thing: The cost of discovering new artifacts and of salvaging artifacts will increase the more artifacts you have. So it is best to salvage ALL bad artifacts and search for better ones! Otherwise you will be stuck with mediocre artifacts, and salvaging one of them will cost 100 diamonds or more!
Book of Shadows is guaranteed to be your 30th artifact if you have yet to acquire it. You need to have 30 active artifacts. Not 25 owned + 5 in graveyard.
Artifact seed is server sided and you will have no idea what artifacts will come next.
General Artifact Tier List is information pulled from here .