Dark Soul Crystal is one of the game's resources, and the most limiting one, bar diamonds. Most players (all but the 300 highest scoring on the battle group of the Fighting Pits) will only receive one of them every three days.
They are obtained through the Fighting Pits, Daily Login Reward, Daily Check in 22 days, Day 7 of Daily Grind, Spending Rewards and also some events (added 2 more in Big Sale). It can be also bought in packs of 2.
See Fighting Pits sections "Rewards" and "Strategy" for tips on getting as many as possible.
They are used to upgrade and research almost everything. In roughly descending order of importance:
- Tower types and skills at the Engineering Quarter
- Magic research at the Magic Ward
- Heroes talents and skills at the Hall of Heroes
- Improved training at the Training Grounds, improved and more supply at the Supply Station (only at first upgrade).
- Rural Camp Research at level 30, 60 and 90
- Researching Elite to Wicked (1) and Wicked to Legendary Equipments (3)
You should probably save DSC to upgrade towers at the start. New tower types are the key to progress in the World. Ditto for the Magic skills. See the comments of Engineering Quarter for some tips on the order of upgrades.
Improved training should probably be next. Once you get to Heroes, concentrate in Hero skills and ignore Hero talents until later in the game, see Honor Star for reasons and related strategy.