While you have no set bonuses equipped (you can get this bonus if set items are equipped as long as no 2-piece or any other set bonus is active) every legendary item increases your damage dealt by 3.75% multiplicatively and reduces all damage taken by 2%. The multiplicative damage effect increases by 3.75% per level to a maximum of 375% at max Gem rank 99.
The secondary effect, unlocked at rank 25, doubles this bonus for a total of 750% damage and 4% damage reduction per Ancient or Primal item. As players in Diablo III can wear a maximum of 13 items, the maximum total bonus from this Gem is 13 x 750% = 9750% damage and 13 x 4% = 52% damage reduction. This effect is very similar to the Legacy of Nightmares set, but it has the benefit of granting you half the bonus for non-ancient items and it only costs you one Gem instead of two Rings. With this Gem equipped it is okay to miss out on a few ancient items as you only lose around 4% damage and 2% toughness if you are missing 1 Ancient Item.
Legacy of Dreams Best Use
Widely used on damage dealing builds, especially on Necromancers:
One of the best Gems in the entire game. This Gem and build style is so popular that people refer to them as “LoD” builds for Legacy of Dreams. So if you hear that saying in the Diablo community, they’re talking about a no-set build based solely on this Gem!
Makes for a very strong Season start as you have a super high chance of dropping this Gem very early on (usually right after Bane of the Powerful). Furthermore this Gem fuels the strongest speedfarming group build in the game the LoD Skeletal Mage “Rat” Necromancer as it allows Necromancers to use extremely powerful ring choices like Krysbin’s Sentence and Convention of Elements with only 3 Elements
Legendary Gem in Diablo 3
Legendary Gem Creation:
If you need extra Legendary Gems to level up, you can make the Rift Guardian in any Greater Rift drop a new Rank 0 Gem by removing all the Legendary Gems with the same name from your Equipment, Inventory and Stash.
The safest way to do that is to put them into the Inventory of different character.