According to my current prototype of difficulty calculator method
HZ-model v.1.0.0, this new quest seems to have a total difficulty of
6523 points.
If you check the summary table I posted
here, this means that
Forsaken Tunnels of Xor-Xel is a little bit more difficult than the average difficulty of KK's quests (5684 points), even higher than ROTWL's quests (6198 points)... and of course the new quest is more difficult than any quest from the Basic Game System questpack.
Attending to the KK chart shown
here, I guess this new quest is a hard challenge for the heroes just before starting the Kellar's Keep questpack since they will not probably face higher difficulties until Quest #3 or Quest #6 of Kellar's Keep and rewards are not too much (250 gold coins, no armory, no potions).