Merlin's power of Infinity is a unique and incredibly powerful ability that allows her to keep her spells active until she herself chooses to dispel them. No matter how powerful the spell, Merlin only needs to cast it once for her power to take effect.
It is extremely versatile and can be used in numerous ways to make her spells everlasting, including freezing others in ice eternally, causing flames to burn endlessly, or even stopping her own time. It allows her to escape the effects of powers which alter her time or her age, such as Grayroad's commandment.
Its effects are so outside the norm of most magic, that it's been called "unfair" and Merlin accused of "cheating" for possessing it.