Sockets become available after a certain level in the game, not sure of the top of my head, 26? Some where after level 20ish. It can be accessed in town at the bottom of your screen. Crest are needed to place in Sockets. You can find them in the Ancient Realm by completing Trials there and opening chests. Other Daily Events can offer them as rewards as well.
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You should fill up your empty slot as soon as you get new crest. Crests will increase your total PWR and are a must for progression in Brave Trials. You can increase the number of slots with gems (20 Gems for 2nd, 30 for 3rd, and 50 for the 4th slot). They will also unlock for free at levels 25, 30, 40, and 50 respectively. You can take them out and switch them around freely. They will never break or go away unless you sell them for coins.
A total of 4 crests can be put into each piece of equipment. They have 5 different rarities ranging from Green, Blue, Purple, Yellow, and Orange which is the best. When you change your gear the crests will stay inside of the sockets and don’t change with the gear. They can be equipped for all of your pets as well. Higher grade crest will offer a considerable increase to PWR. Since results are random though things can get better or worse pretty easy. If you like the stats rolled you can lock the socket so it wont change when re-rolling.
You can re-roll the stats on crests 3 times a day for free. Then you have to pay with gems or medals. The results will be random with each roll. Unless you suffer a large reduction in power on a re-roll gems are better spent other places like shards. Once things get filled up you should trade in all your weaker crest for coin.
Brave Trials Wiki & Guide – Classes – Faen List – Rebirth Faen – Best Faen – Grade 6 Faen (G6) – Daily Events – Ancient Realm – City Defense – Vault Boss – Excavate – Socket – Star Levels & Benefits – Tips, Tricks, Hints, & Cheats – Walkthrough – BlueStacks & AndY