What was the die size?
1d6, with each side adding its numbers. This does mean that a group of 5 adventurers would be on even footing with a 5-strong tribe. Adventurers are deadly! They just got unlucky here, also the fact that the only entrance to the dungeon leads right to the Toadmen's doorstep.In the next year, the Toadmen swarmed the surface. They razed a shepherds' hovel but fell back once the Theliiri started putting up resistance. Both sides bore some losses, then the Toads decided to look for easier prey.
Deep in the lower tunnels, their scouting party entered a hall of statues. Depicting fearsome dwarven warriors clad in metal armor, they seemed to stand in a circle, as if guarding something. The scouts ignored them and started rummaging through the hall in search of useful loot, salvageable bronze and golden trinkets they were used to finding in abandoned halls. Nothing. Undeterred, they went downstairs, breaking an old grate to find a dusty, abandoned workshop. Bingo!
Then the "statues" came to life.
***
Despite unsuccessful brushes with the Theliiri and the loss of a scouting team at the hands of the Warped, the Toadmen continued to multiply. Though not sophisticated enough to make their own metal tools, they proved adept at salvaging whatever they could find, and ever so greedy for jewels, territory and fresh meat. The spiders in the spider cave became food, and so did the bats.
In the next season, they fended off an attack from the Antlings (who, having found a treasury, concluded this place might bear many useful secrets, and sent back to the Hive for reinforcement). Some of the Antlings' newfound loot thus ended up in the Toadmen's slimy hands. Subsequently they were again attacked by the Warped, losing more warriors but managing to beat the abominations back into the lower tunnels. As the lower tunnels were being slowly flooded, contact with the enemy was lost.
(The Toadmen can move through water with impunity, but the Warped cannot).The Toadmen now had enough loot, and enough other tribes knew this, that rumor of wealth hidden in the tunnels started spreading with renewed strength, both on the surface and in the underworld.
***
Other groups haven't had such spectacular times.
The newly-arrived human miners dug a shaft down, reaching the underworld in the second season but finding nothing but a plagued cave.
The Gray Behemoth spent a season hunting, using gems to lure prey to its lair. Eventually hunger drove it to move further down, towards the ruins of Tursong and the horror-blobs inhabiting it.
That was quick. The Age of Villainy is right around the corner is now possible (but not guaranteed). First, though, I need to finish the year.