This achievement is similar to the achievement from the last level, The Stones' Power. Each time you conquer a graveyard with Thalya, it will spawn undead. Look to the upper left of the screen and you'll see Thalya's icon and another one that looks like a zombie hand bursting out of the ground. Click the left stick, select the second icon, and press A. Like with the Gehenna Stones' magic, it needs to recharge after each summoning. Each graveyard lets you summon up to 7 zombies at a time, for a total of 21 when all three are captured and fully charged. Unlike the Gehenna Stones, you don't have to wait for it to be completely full. You can activate it whenever you want. The achievement will pop immediately once you've summoned 98 undead. Just don't end the level until the achievement pops and you're good.