Does the text under teh Epic Spells section help to explain how things eork?
From SRD:
From SRD:
EPIC SPELLCASTING
Once an epic spell is developed, the caster knows the spell. A developed epic spell becomes an indelible part of the caster and may be prepared without a spellbook (if a wizard is the caster). Characters who cast spells spontaneously, such as sorcerers, can cast a developed epic spell by using any open epic spell slot. Druids, clerics, and similar spellcasters can likewise prepare epic spells using epic spell slots.
A spellcaster can prepare or cast any epic spell he or she knows as many times per day as he or she has available epic spell slots. A spellcaster who can cast epic spells has a number of open epic spell slots per day equal to one-tenth his or her ranks in the Knowledge skill appropriate to the spell and the caster’s class. Knowledge (arcana) is appropriate for arcane casters, and Knowledge (religion) or Knowledge (nature) is appropriate for divine casters. The rules for rest between casting a day’s allotment of epic spells are the same as for rest required to prepare standard spells. If the caster doesn’t use up a day’s allotment of epic spell slots, the unused slots remain available whether or not the spellcaster receives appropriate rest.
Even if the epic spell has been developed and an epic spell slot is available, successfully casting an epic spell isn’t assured. The caster’s Spellcraft skill modifier is vital for casting an epic spell. To cast an epic spell, a spellcaster makes a Spellcraft check against the epic spell’s Spellcraft DC. If the check succeeds, the spell is cast. If the caster fails the check, the epic spell fizzles and the epic spell slot is used for the day.
Because epic spells require Spellcraft checks, a spell is beyond the caster’s ability if the final Spellcraft DC is greater than 20 + the spellcaster’s Spellcraft modifier. Epic spells with DCs higher than 10 + the spellcaster’s Spellcraft modifier are risky; a caster can take 10 when casting an epic spell, but he or she can’t take 20. When routinely casting epic spells, most spellcasters take 10 on their Spellcraft checks.