The Chain Spell feat (Complete Arcane p76) can be used with:
Any spell that specifies a single target and has a range greater than touch…
What counts as a spell that specifies a single target? Does a ray that can only affect one creature count? What about a spell that can only affect one creature if cast at a particular caster level? (1 creature per 5 levels for example)
Best Answer
The only spells that are valid for use with Chain Spell are targeted spells that only target a single creature and have a range greater than Touch. A targeted spell is one with a "Target" line in its description.
As a few examples, feeblemind is valid because its description says:
Or blindness/deafness:
Other spells which affect a single subject but are not targeted, such as enervation, cannot be used with Chain Spell. Nor can spells like slay living which target a single creature but have a range of Touch.
This was clarified in a FAQ entry on 5/20/2008, though it's no longer directly available due to several revisions to Wizards' website:
Spells can have one of three "what it does" parts to their description: Effect, Area, and Target.
Effect spells bring something into existence, however briefly, such as a summoned creature, wall, or a ray. Directly offensive Effect spells generally require attack rolls and don't allow saves to resist them. They can also be aimed at creatures you cannot perceive, provided you pick the right square.
Area spells affect the entire indicated area/volume, barring obstructions, regardless of your ability to perceive creatures or objects in the area. They generally allow saves and don't require attack rolls.
Target spells directly target specific subjects and don't require attack rolls unless their range is Touch. You cannot cast them on creatures you cannot perceive, even if you know what square they are in, unless you can also touch them.