Are you confused about where and for what to use your Diamonds? here is a quick guide :
-Abyss Party: Abyss Party is a good way to get more BP and in order to enter Abyss Party you need abyss tickets. abyss Tickets are buyable for 30 diamonds each. Using diamonds to get abyss tickets is an efficient way good to get abyss gear ( that you can dismantle to get abyss stones to upgrade your abyss gear ).
NB: You could also get an abyss visa or abyss Green card from abyss party which gives you more abyss party tickets.
-Mercenary System: by doing Mercenary Hire you can get items such as Abyss EXP codex, Candy, Weak cake, Titles etc... There are Levels of Mercenaries: SS, S, A, B, C, D. The Highest level is SS and the lowest is level D. You can also buy mercenary acceleration order for 30 diamonds each from Grocer to speed up the process.
-Player Kill or PK: Player kill is a good way to show your Dominance... and to get some good stuff. When doing PK you can get False Gold Trial Ticket and Holy Keys from certain players. Holy Keys are used to Open Relic Chests where you can get some items. Doing a lot of PK can get you trophies that you can use to upgrade your Military rank and you get BP from upgrading your military rank as well as an SSR Title Assassin once you get your Military Rank Level to 810. In order to do more PK, you have to click Senior Secret Agent in order to refresh your list to do PK. Its Starts from 480 and It increases every time you refresh the list.
-Secret Treasure Bay: From Treasure Bay, you can get valuable items at a low price of Zircoins or Diamonds. It resets the items every week.
-Entertaining Guests in Maid Cafe: By Entertaining Guests in Maid Cafe for few diamonds, you get a lot of Exp. You can do it 5 times a day. At the start, it costs 70 and from there 75 to 80 to 85 to 90 and ends at 95. As you entertain guests you can increase the amount of Exp you receive by entertaining Guests every day. Every day it boosts the amount of Exp you get over 2% and the max % is 50. Once you entertain Guests for continuous 25 days it will reach 50%, if you entertain guests after it reaches 50%it the boost won't be increased anymore but will stay at 50%.
-Pet Draws: Here you get Pets of all rarities. There are 4 levels SSS > SSR > SR > R. SSS(very rare) is the strongest and R (common) is the weakest. You can do draws with Royal Cat Food or 740 Diamonds. From doing draws you can get pets that you didn't already have and can increase your BP. Once you do 500 pet draws in total, you will get an SSR Mage Character Road.