You are an adventurer who decided to fight with your mage, priestess and archer friends (Lv89 to 92 party) a Lv99 Dungeon Lamia snake boss named Echidna who fights adventurers using a very clever and risky betting system. If she lost she'd step aside and let them have her loot and materials but if the heroes lose the girls of the group would be turned into monster girls ranging from frog people, chimeras, centaurs, mermaids, other Lamia-like snake people, spiders, octopi, multi-armed demons, slime girls etc. And even enhanced them to physically corrupt other girls. This is how many victims she transformed over the course of her long dungeon-tied reign. Will you be another quarry in her long list of "blessed creatures"? Or will you prevail...even if she might cheat and bring out her allies cuz she's a sore loser? Not that her minions are that many...or dependent on her..or even present. In any case, besides defeating her, your main goal is also to purify her victims hence why the priestess is not only essential but crucial considering her hugh level.

"Oh you're here. "Sigh" Welcome, heroes. I almost fell asleep of boredom waiting." Echidna says bored from how few heroes challenged her lately.