The grell from LoM is the same as the MM II converted monster, except:
[sblock]+5 to hit with tentacle instead of +4
+0 to hit with bite instead of -1 and 1d6 damage instead of 2d4
carries a lightning lance (+5 ranged touch within 60 feet, 3d6 electricity damage (no save))
10 foot reach with tentacle
Improved Grab for medium creatures or smaller, +14 Grp, 1d6+1 Constrict damage
Can threaten others while grappled by taking a -10 penalty instead of -20 to Grp checks
Paralysis DC is 10+1 for every additional tentacle hit - (single save instead of multiple saves)
Paralysis duration is 2d4 rounds instead of 4 rounds
Immune to gaze attacks, visual effects, illusions, and other sight-related attacks
Tentacles have 5 hp instead of 10 for sunder purposes - severing a tentacle deals 2 damage
Has Weapon Finesse feat in addition to Flyby Attack
Hide +8, Listen +4, Move Silently +8, Spot +4[/sblock]