I don't know if this belongs here, but it seems like as good a place as any.
The Situation
The wizard stares intently at the cracked, evil visage of the lich-king. As he looks around at his companions, he knows that they don't stand a chance without him. They are powerful, yes, but against such a mighty spell-caster, only he holds the key to their success. With determination etched on his face, he casts the Mass Haste spell that he prepared for the occassion. He revels in the power of magic as his every action is accelerated to a blur of speed. Then he goes for what he hopes will be the beginning of the end for the lich-king as arcane words of power fill the air. With satisfaction he looks around at the effects of the Time Stop. His companions' expressions are frozen on their faces. A droplet of water hangs motionless in midair, just inches from the stalactite from which it just fell. As the wizard prepares to unleash his full fury upon the undead abomination, his jaw drops in horror. The lich-king turns its head toward him, its eyes burning points of hatred, and smiles. Eldritch magic spills from its lips, and the wizard screams, knowing that only the lich can hear.
What happened
The lich-king has epic levels, and he took the feat
Spell Stowaway. The spell he chose, of course, was Time Stop. Too bad for the wizard, as now it's the lich's turn.
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Piratecat, I don't know if your players ever use the Time Stop spell (I haven't noticed it yet), but this would be a great surprise to spring. Personally, I can't wait until my players reach that level...