Planar Binding Spell
D&D Description of the Planar Binding Spell : This spell functions like lesser planar binding, except that you may call a single creature of 18 HD or less, or up to three creatures of the same kind whose Hit Dice total no more than 18. Each creature gets a saving throw, makes independent attempts to escape, and must be persuaded to aid you individually.
Deeper info on this Dungeons & Dragons Planar Binding Spell