From Dragon Mania Legends (DML) Wiki
Sigil Energy is the energy used on the Campaign Map for the Sigil Campaign each time a battle is fought. Sigil Energy regenerates automatically at 4 units every 24 hours, up to the maximum base amount of 4. It can also be recovered for the amount of 80 Gems for all 4 Sigil Energy, or 20 Gems per Sigil Energy if less than 4 is already used. Sigil Energy can be refilled by pressing the + button while on the Sigil Campaign, or by trying to engage a battle without any energy left (which will bring up the refill screen from where the energy can be refilled with Gems).
It costs 1 unit of energy to engage in a battle on the Sigil Campaign.
See Battle Energy for more information