The sand inside the thorns contains a total of 26 manticores: AC4, HD6+1, hp 21, 25, 35, 32, At 2 claws/1 bite or spikes, D1-4/1-4/2-8 or 1-6, save F6, C. A manticore can shoot up to six spikes four times. They will attack in as small a number as possible, but with a large chance of winning.
Also, there are three chimeras who will only come if needed: AC4, HD9, hp 52, 38, 47, At 2 claws/3 heads (goat gore/lion bite/dragon bite or fire), D1-3/1-3/(2-8/2-8/3-12 or 3-18), save F9, C. The dragon head can breath fire up to three times.
The lake is an illusion showing a lake covered with muck, seaweed, and corpses of animals and humans. Actually, it is a volcano, home of the evil wizard Grabdok.