Codex of Power is an exciting new reward system that is tied to completing dungeons in Diablo 4. This reward system is based on a tiered system, where you collect Codex Fragments by defeating enemies, completing objectives, and killing bosses in dungeons. For every Codex Fragment you collect, you will receive rewards such as legendary items, skill points, or other valuable resources. The more Codex Fragments you collect, the better rewards you will be able to claim. 

The Codex of Power is broken down into four tiers: Common, Sacred, Ancestral, and Mythic. Each tier has its own rewards, with the Mythic tier being the most powerful. As you progress through each tier, the rewards will become more powerful and valuable. For example, on the Ancestral tier, you can receive legendary items, skill points, and even permanent bonuses to your stats. 

Diablo 4's Endgame Dungeons are the ultimate challenge for players looking to push their limits and test their skills. With 115 dungeons in total, each offering a unique experience, players will have plenty of opportunities to explore the depths of the world and take on the toughest of challenges.

In addition to the standard rewards of experience and Diablo 4 gold, completing these dungeons will unlock a legendary aspect in the Codex of Power. This aspect is permanently unlocked for the account and can be applied to any rare or legendary pieces of gear. It is also infinitely reusable, meaning that players don't need to re-earn it. This will be a great way to gear up in the early and mid-game and will continue to be a useful tool even in the endgame.

Furthermore, the first time a player completes a dungeon they will earn 20 renown in the zone. The Renown system is a complete system for each area and unlocking certain amounts of renown will give players account-wide buffs. These could include additional potion slots, a primary stat increase, and other rewards.

Finally, completing dungeons will give players plenty of loot. They'll receive gold, experience, and loot from enemies, breakable objects, and of course, bosses. There's also the Nightmare Sigil system which amplifies the amount of loot players receive from each dungeon. Also, developers have hinted that certain dungeons may have a higher chance of dropping specific gear slots.

Overall, Diablo 4's endgame dungeons are sure to provide hours of fun for players looking for the ultimate challenge. With 115 dungeons, account-wide rewards, and plenty of loot, these dungeons have something for everyone. So, if you're up for a challenge, be sure to check them out.