The Elder Rift and Challenge Rift are located in the Palace Courtyard area of northern Westmarch.
As you progress through the main story you will be taken there, but you can find both Rifts side-by-side in the middle of the square.

To the left-hand side of the Elder Rift and Challenge Rift you will find two important vendors: the Crest Merchant and the Fading Ember Ember Merchant. These two shopkeepers perform important roles and you will want to remember they’re there.

What is the Elder Rift in Diablo Immortal?
The Elder Rift is Diablo Immortal’s premier source of some of the game’s most powerful items: Legendary Gems. It’s also one of the places where you do the most dungeon-crawling.
When you enter the Elder Rift you fight through the generated dungeon with a specific task to complete, this can be things like “defeat enemies quickly”. As you fulfil the requirements of the task, a bar on the left-hand side of your screen will fill.
Once the bar is fully completed the Rift Guardian will appear. This is the boss enemy of the Elder Rift. Defeat them and the dungeon is completed!
A Legendary Crest being used to open the Elder Rift in Diablo Immortal
The rewards you get for completing the Elder Rift vary depending on the kind of Crest you use to open the rift.
Opening the rift with no Crests will still yield good items, XP points and Fading Ember - a resource you can trade to the nearby merchant for Runes.
Adding a Rare Crest to the Elder Rift will add random Runes to your reward pool. Runes are used to craft Gems. There is also a very small chance to get a Legendary Gem from a Rare Crest.
Finally, adding a Legendary Crest to the Elder Rift will guarantee you a random Legendary Gem upon completion of the dungeon. Since they are so sought after, this is the top prize and worth setting your sights on.
However, Legendary Crests are very difficult to obtain without spending real money.