Heal is a self-effecting spell cast by monsters such as Council Members and Cantors. As the name implies, Heal heals the caster for a portion of damage.
In past versions of the game, some monsters (especially Council Members) would flee to safety when at low health and frantically cast Heal. This allowed them to fully recover from their injuries, making quests as The Blackened Temple (quest) lengthy and arduous tasks.
In Diablo Immortal, Overseers found in Destruction's End use this skill when they are not being attacked and are far from the player.
Nowadays Heal is no longer a real issue. Monsters don't heal as often as before and player characters should be able to out-damage the healing with relative ease.
Damaging Skills
Fire — Fire Arrow • Fireball • Fire Bolt • Fire Hit • Firestorm • Fire Wall • Flame Circle • Hydra • Incineration Nova • Inferno • Meteor
Ice — Arctic Blast • Blizzard • Cold Touch • Fire Hit (Cold) • Freezing Charge • Glacial Spike • Hoarfrost
Poison — Poison Bolt • Poison Breath • Poison Cloud • Poison Spit • Poison Spray • Poison Sting • Poison Strike
Lightning — Lightning • Lightning Arrow • Lightning Globe • Red Lightning Hose • Shocking Hit • Spark Defense
Physical — Blow Dart • Charge • Corpse Cannon • Destructive Strike • Ground Tentacle • Leap • Leech-life Ball • Quill Dart • Slam
Magic — Bone Spirit • Hellish Projectile • Unholy Bolt
Non-damaging Skills
Indirect Attack — Blood Mana • Curses • Mana Rift • Minion Frenzy • Putrify • Spin Web • Stun
Spawning — Bone Prison • Festering Appendages • Lay Egg • Revive • Summon Minion
Self-effect — Bone Armor • Burrow • Fly • Frenzy • Heal • Teleport • Vile Effigy