Warmth is a base Fire Spells skill that passively increases the Sorceress' mana regeneration rate. From the very beginning, this can be a very useful skill to invest a point or two in, as it increases your ability to cast more spells without using potions as often.
Tips[]
Consider only adding one point to this skill. Anymore might be useless, as players often find many items adding to total Sorceress skills (items that have +x to all skills or +x to all Sorceress skills or in this case +x to fire skills). Since you only need a point or two in this skill, the additional +skills from your accumulated items later in play will more than make this skill function adequately. Maxing this skill is not adviseable.
Lore[]
If a Sorceress truly wishes to become a formidable spellcaster, she will pursue this talent. By collecting the essence of the ambient heat in the surrounding air, she can convert this energy into the mystical force that powers her spells. In this way, she recovers more quickly from her magical exertions.[1]
References[]
↑Diablo II Manual, page 85. Blizzard Entertainment, 2000.