Boss tokens used to enter bosses spawns you have to put the token on the coal basin and step on the blue mystic flame.

These tokens are buyable from the online shop each 3 for 5 points.

Obtainable by:

Boss token quest it's reward is 5 boss tokens and strong mana potion.

Rain item event

You can change 100 undead hearts for 1 boss token at undead merchant NPC.