A 15-foot-radius sphere of dark fog, filled with the necromantic Miasma, appears centered on a point you can see within range. The fog spreads around corners, and its area is heavily obscured. It lasts for the duration, or until strong wind disperses the fog, ending the spell.   When you cast this spell, you can empower it with your own life essence by losing 10, 15, or 20 hit points when you cast it. When you do so, treat the spell slot used to cast the spell as being one level higher if you lost 10 hit points, two levels higher if you lost 15 hit points, and three levels higher if you lost 20 hit points, up to a maximum of 9th level.   When a creature enters the area for the first time on a turn or starts its turn there, it must make a Constitution saving throw, taking 3d8 necrotic damage on a failed save.   Additionally, when a creature tries to leave the sphere of fog, you can choose for the fog to grasp the creature with misty tendrils. The creature must make a Strength saving throw to leave the fog, and is unable to move on a failed save.