There are some sorcerers who, like some of their wizard counterparts, exchange access to the full spectrum of magic for greater power in their chosen focus. One such way of doing this is by tapping into the raw power of an element: Earth, Air, Water, and Fire. If this is the case, they can only cast spells associated with that particular element and no other spells (including non-elemental spells).
It is up to the DM whether a spell can be considered elemental in nature (or if an elemental version can be known). In general, such a sorcerer may discover elemental variations of non-elemental spells (e.g. magic missile) that must be cast as spells one level higher than normal. Such sorcerers may also cast non-sorcerer spells (e.g. druid or cleric spells – even domain spells) that are associated with their element so long as they are ‘pure’ spells (i.e. they do only and purely elemental damage; not, for example, fire and holy damage, or holy fire damage) and so long as they are cast at one spell level higher than normal, domain specific spells forcing one additional level increase. At the DM’s discretion, impurely elemental non-sorcerer spells (such as a mixed damage spell, or a spell that might fit within the purview of an elemental type) may be learned, but they must be cast as spells 2 levels higher; these are cast as a strictly elemental variant of the other spell. At the DM’s discretion, non-elemental domain spell variants may be discovered, but these should require at least a 3-level increase.
Should a sorcerer be an elemental sorcerer, he may gain benefits analagous to a specialty wizard, namely he casts all spells (since they’re all elemental) at a +1 bonus, can know one more spell of each level that he can cast than other sorcerers of his level, and can cast 1 more spell per day per level that he can cast than other sorcerers of his level.
If the elemental sorcerer gains access to a class feature, feat, or other ability that would enhance the effects of a specialist in exchange for further limitations, such as the Enhanced Specialization class feature of a Red Wizard (DMG 193), they instead give up access to non-arcane variants but otherwise follow the description given in the feat, feature, etc. For the purpose of feats, features, skills, and the like, an elemental sorcerer’s elemental focus counts as a school of magic that they have specialized in.
Just as a specialist wizard cannot multi-class as another wizard (e.g. wizard [conjurer] 5, wizard 1), niether can an elemental sorcerer multiclass as another sorcerer (elemental or otherwise). Furthermore, just as specialist wizards cannot cast spells forbidden to them (by their forbidden schools), elemental sorcerers can never cast spells of an opposing element, even if they would gain access to such spells by multiclassing as another spellcaster (assuming the DM permits specialists to multiclass as spellcasters).
Elemental sorcerers further gain resistance to their chosen element equal to their caster level, which stacks with any other sources of elemental resistances of the same type, and can be further enhanced by magical effects. For example, if an aasimar who has resistance to electricity 5 became an Air Sorcerer, he would have resistance to cold and acid 5, and resistance to electricity 6 at level 1; his resistance (electricity) would increase to 25 at level 20 as he gained levels as an elemental sorcerer. An elemental sorcerer can suppress this ability as a free action, and resume it as a standard action. The example assumes that Electricity associates with Air, Cold associates with Water, Acid associates with Earth, and Sonic energy is unassociated (Fire should be obvious); the DM may rule otherwise and should specify as soon as possible.
Special: as devotees of an element, these sorcerer variants gain an inherent +2 skill modifier to any diplomacy check made when dealing with a creature whose type or subtype matches the sorcerer’s preferred element, or is close to it (e.g. a fire sorcerer dealing with a magma mephit), and an inherent +2 skill modifier to any intimidate check if the creature is elementally opposed (e.g. a water sorcerer dealing with a magma mephit).
The elemental sorcerer’s familiar must be of the same type or subtype as his chosen element, in addition to any other requirements, but he can gain advanced familiars 1 level sooner than normal sorcerers. If he takes the Leadership feat, his leadership score when attracting a cohort is modified by -1 if the subject’s type or subtype does not match his chosen element. At the DM’s discretion this may replace or diminish the effects of alignment on leadership score.