At 2nd level, you learn how to use a bound vestige to boost your arcane spellcasting. With this ability, you can choose to forego gaining one of the vestige’s granted abilities in order to gain one additional arcane spell slot per day of any level up to the highest you can cast. Thus, if you bind the vestige Dantalion (see page 28), you could give up the read thoughts ability he grants you in exchange for a bonus spell slot that day.
You decide which ability to give up before you make the binding check, and you can forego only one ability per day in this manner, even if you can bind multiple vestiges. Vestiges react poorly to your use of this ability, so you take a —5 penalty on your binding check. If you make a good pact despite this penalty, you gain a bonus spell slot in exchange for the ability you gave up. If you make a poor pact (that is, you fail your binding check), you do not gain the extra spell slot, and you still give up the designated ability for the day.
When you use this ability and make a good pact, you sever a portion of the bound vestige’s awareness of the world that it would normally receive through you. Essentially, your binding amounts to a certain amount of trickery. The vestige being bound expects to be able to experience life through its binder, but when you make a good pact, you partially suppresses the benefit the vestige receives, making it uncomfortable and angry.