If you take Yoshimo to Spellhold, he will betray you. After defeating Irenicus for the first time, Yoshimo and several assassins will attack you. After you kill Yoshimo, he says he wants you to bring his heart to a temple of Ilmater. You just have to take his heart and bring it to either the temple in Waukeen's Promenade or the temple in the Slums District. Talk with the priest, give him the heart, and you'll give eternal rest to Yoshimo. This will get you a whopping 200,000 experience. Kind of pays off for the betrayal, don't you think?