Night Mare runs fast and jumps far frequently, making her hard to target. After running and jumping around for 10 seconds, it will charge at you, causing much more damage then before. After doing so, it will create 5 Demon Sickles (similar to Demons) and shoot them off. Rinse and repeat.
Night Mare's charge goes twice as long in Expert Mode. It creates 9 Demon Sickles instead of 5. At 12,000 HP, 8,000 HP, 6,000 HP, 4,000 HP and 2,000 HP it creates Vile Leechweeds on the ground. If you touch these, it will do 40 damage and heal Night Mare by 120HP.
13-18 Greater Healing Potions (100%)
22-28 Souls of Night (100%)
Horseshoe Bow (33.34%)
Unholy Arrows (100% with Horseshoe Bow)
Demon Rapier (33.33%)
Cursed Apple (33.33%)
Night Mare Trophy (10%)
Night Mare Mask (14.49%)
Treasure Bag (100% in Expert Mode)