Templates are a concept introduced in Dungeons & Dragons 3rd Edition, designed as a way for DMs to modify creatures without accidently making an overpowered TPK machine. The basic idea is, for example, any mortal can become a vampire, so instead of just leaving the DM to figure out how that affects their hit points and base attack bonus, WotC provided Templates in most Monster Manuals that could be added on to existing stat blocks to turn your 12th level dwarf ranger into a vampire.
3.5 & Pathfinder also continued this tradition.
List of Templates
Monster Manual 1
- Half-Dragon: Of virutally every color
- Celestial Creature
- Fiendish Creature