Here is another monster I have converted to the Classic D&D rules:

Naliseth
Armor Class: -2
Hit Dice: 10* (L)
Move: 240' (80')
Attacks: 1 Bite
Damage: 3d6
No. Appearing: 1
Save As: F10
Morale: 10
Treasure Type: D
Intelligence: 6
Alignment: Chaotic
XP Value: 1750

Monster Type: Lowlife (Very Rare)

These huge Sea Serpents are believed to be the result of experiments by the Egg of Coot. A character killed by the Naliseth will rise as a kind of undead called the Spawn of Naliseth.

The Naliseth first appeared in Dave Arneson's Blackmoor (3E).

-Havard

Adding in some details from the 4e book and borrowing the swalowing mechanics of ye old purple worm..

These huge Sea Serpents are believed to be the result of experiments by the Egg of Coot. They are bottom feeding, eel – like beasts with vestigial flippers, razor sharp spines and an impossibly dense and deadly set of spike like teeth in an oversized mouth. Any hit more than 2 over the minimum necessary to hit indicates the Naliseth has swallowed its victim. The swallowed victim will take 15 points of damage (including 5 from acid and 5 from poison) per round. The victim may make melee attacks while inside and can cut themselves out if the Naliseth is dead. A dead victim will be digested and unrecoverable in 12 turns. The blood of the Naliseth is poisonous and anyone exposed to it must save versus poison or take 5 hp of damage per round until cured or dead. A character killed by Naliseth blood but not swallowed and digested will rise as a kind of undead called the Spawn of Naliseth.

Great additions!
The swallow ability does seem appropriate. If we include that, the XP value should increase though. I also thought about doing different sizes of this creature.

-Havard