In game currency.
Collected by killing enemies, the more creative the higher the reward.
Amount differs by type of enemy, generally the harder to kill the more they are worth.
A combo will result in higher rewards.
Sometimes enemies drop coin when killed.
Mr. Moneybags always drops 500 coin when killed.
Also known as "Arcane Coin" "Rift Coin" and "Rune Coin".
A theory surrounded having to spend coin on traps to defend the rift is that the Order purposefully opened the rifts to allow enemies through in order to create a reason for traps to exist. The Enemies have been paid by the Order to invade and the Order overcharges for its trap rental assuming you the player are able to overcome the enemy.