Here is the full list of all 21 Necromancer Returns achievements.
Learn all skills.
Complete all quests.
Summon 5 skeletons.
Use units’ abilities 20 times.
Use Vampirism spell 5 times.
Heal 1000 of health.
Spend 10000 gold.
Resurrect after death.
Put artifacts on the hero.
Win 40 battles.
Deal 15000 of physical damage.
Deal 5000 of magical damage.
Win 20 battles.
Lose 2000 of health.
Deal 5000 of physical damage.
Spend 200 of mana.
Win in 5 battles.
Complete Tutorial.
Upgrade all units.
Complete campaign mode.
Win in 5 battles in the mode of "Survival"