World Serpents Grasp
Whenever you hit a slowed or immobilized target with an attack, you can knock it prone
you are immobilized (among others)
The DM maintains that WSG does not trigger in the case. What say You?
The proper definition of Restrained does not include Immobilized or Slow in any way and therefore World Serpent's Grasp will not trigger.
A Restrained creature:
The Rules Compendium (pg 233) goes on to note that
Based on this it would not be unreasonable to make a ruling that World Serpent's Grasp did affect restrained creatures, but it isn't supported within the rules as written and would be a house rule.
This question is reminiscent of one previously answered:
The same standard seems to apply: Restrained is not Immobilized and thus is not a valid condition.