I started GMing my first sorcerer about 5 sessions ago. After several readings of Skelos and Core, and making several rulings on spells during gameplay, I now at last feel comfortable with this section of the rules. This was probably the most difficult area of the rulebook for me to conceptualize. So, here’s my explanation:
Enemies will often spend Doom in order to cast spells, just the way other enemies may spend Doom to activate their most powerful combat abilities. This is just Doom acting as the GM’s “Ammo.” In general, player characters are not required to spend Doom to cast. If a specific spell requires Doom to be spent, it will say so in the text for that spell.
PC sorcerers are likely, however, to exchange Doom for bonus die on the skill test to increase their chances of success. They are also likely to accidentally add a lot of Doom all at once since any die that does not produce a success on the Sorcery test results in a complication, and a 20 results in two complications. I believe the Doom cost for NPC sorcerers is, in part, an abstraction of the probability that the sorcerer will produce at least one complication.
The steps for a PC casting a spell are as follows:
Spend a minor action to “Focus.” This is a unique action only taken by sorcerers when casting a spell.
(Note that although on Core p 170 the wording indicates you “must” Focus as the first step, later in the passage it indicates you may voluntarily skip this step. "If a sorcerer chooses not to Focus (or fails somehow) the sorcerer increases the chances of additional Complications: instead of occurring on a roll of 20, an additional Complication will ensue on a roll of 19 or 20.)
The sorcerer loses 1 Resolve. Some spells may have additional costs, including additional Resolve loss, but all spellcasting costs 1 Resolve just to attempt, even if the spell fails.
Next, a standard action is used to attempt a Sorcery test. The Difficulty is determined in the spell entry and may depend on the specific effect the sorcerer is attempting to use use.
The sorcerer may spend Momentum to alter or enhance the effects of the spell. The rules do not seem to specify that this Momentum must have been generated by the Sorcery test. I believe Momentum stored in the pool may be used for this purpose.
Here’s where it gets a little tricky, because the wording is pretty confusing. Some spells require, as part of their casting, a “secondary action,” according to p 170 of Core. The player will have to check the text to see if this is required for the particular spell being attempted. Since a Minor and a Standard have already been spent, they’ll need to spend Fortune or Momentum for a Swift action to attempt this, or else wait for their next turn to continue the spellcasting process. If this next action fails, I assume the spell fails. EDIT: One of my players cleared this up for me, using an effect of Enslave as an example. As I understand it now, after the spell is cast certain, effects might require additional checks to take place. These are presented slightly out of sequence in the rules.
Finally, the spell is ready to be cast, but for most spells, causing the spell effect to actually take place requires another Standard action. Once again, some spells may specify in their body of text whether the spell is used without spending this action. Otherwise, the sorcerer must wait until their next turn before “using” the spell, allowing its effects to manifest. In order to avoid the wait, the sorcerer may spend 2 Momentum or a Fortune point for a Swift action to use the spell immediately.
The one thing I am not totally sure on is whether steps 5 and 6 are two separate actions, or if they are just referring to the same thing and the rules just got too verbose for their own good. Just do the homework and make sure you and the player know how many actions they must spend to cast their spell. My guess is few if any spells require anything for step 5. I’ve read all the spells in Core at least 3 times, but without putting what I’ve read into regular practice have retained very little from the exercise.
So, to sum up the cost, a typical spell cast by a player character will typically involve a massive expenditure of Momentum, and possibly 1 or more Fortune Points. Doom is very likely to be added to the pool, likely in large amounts, but this is not necessarily a prerequisite for casting the spell; it’s a by-product of all the things likely to go wrong during the process.
Then, the rules for Consequences. If the GM wishes, a player character sorcerer may ask to invoke this rule. The spell is successfully cast regardless of the results of the Sorcery test in step 3. As with normal spellcasting, every die that does not produce a success results in a Complication, and any complications naturally rolled (20 if the sorcerer Focused before casting, 19-20 if not), produces two complications. To make this method more dangerous, however, the player declares any Momentum spends before making the roll, and every point of Momentum they fail to generate also results in a complication.
Then the rules go on to remind you that you can get screwed over by your own spell. The Horror you summoned can turn on you, the rockslide you caused can bury you. A good way for the GM to spend that hill of Doom you just gave him.