Raw mana nodes can be transformed into a particular kind of mana by adepts, mages, and archmages. You must have at least one kind of mana to promote units with spells from that "school." For example, Death 1 grants a spell to an arcane caster that summons a skeleton, and you must have at least 1 Death mana to promote a unit to Death 1. For the purposes of referring to spellcasters here, please note I am referring only to arcane spellcasters. Divine spellcasters follow different rules.
If you have 2 of a given mana, any arcane units you produce when you have those 2 mana automatically start with the level 1 promotion associated with that school. If you have 3, mages will be granted the automatic level 2 promotion, and with 4, archmages will gain the automatic level 3 promotion. These promotions do not cost experience points, and indeed, whenever you create an arcane caster or upgrade one to the next level, you get one EXP-free promotion. Mages and archmages cannot be built (except for certain heroes), they must be "upgraded" from adepts with a certain amount of EXP (archmages are upgraded from very high EXP mages and you can only have 4). Only mages and archmages have access to level 2 spells, and only archmages have access to level 3 spells. You also need certain technologies to unlock mages and archmages, Sorcery for mages, and Strength of Will (VERY expensive tech) for archmages? It should be noted there are some heroes that are archmages and may come with earlier, cheaper techs, but they may be religion or civilization specific.
Since getting EXP for adepts is understandably hard, given their low strength, the "channeling" promotion that all casters begin with has a chance to passively grant 1 EXP each turn (note: this is far from guaranteed). You might also look for weak barbarians to use for EXP farming--you can weaken them with catapults or something before sending in the casters. For these reasons, if you plan on using casters in numbers, it's best to build them early, so they can start building EXP right away.
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