Vile Effigy is a duplication summon cast by the boss Baal to create a clone of himself.
Baal's clone deals the same amount of damage as the original, but is easier to kill. It may not be worth the time to even bother with it however, as it also has a duration if a player (or group of players) return to town and wait for a minute or two for it to expire, at which point you can turn around and come back to continue fighting the Lord of Destruction. Sometimes Baal will cast another Vile Effigy as soon as you return, but a Scroll of Town Portal is pretty cheap, so just open another one and try again until he stops cloning himself.
The clone is created with the same percentage of remaining life, and looks identical to the real Baal, making them difficult to tell apart, but not impossible. The easiest method to tell them apart is to look at their Life bars and their label of Demon below it. The fake Baal's Demon is perfectly in line with his name, whereas the real Baal's Demon is offset to the right.
When Vile Effigy is cast, Baal will announce it by saying, "My brothers will not have died in vain..."
Damaging Skills
Fire — Apocalypse • Eruption • Fire Arrow • Fireball • Fire Bolt • Fire Hit • Firestorm • Fire Wall • Flame Circle • Grenade • Hydra • Incinerate • Incineration Nova • Inferno • Meteor • Minefield
Ice — Arctic Blast • Blizzard • Cold Touch • Fire Hit (Cold) • Freezing Charge • Frost Bolt • Glacial Spike • Hoarfrost • Ice Shards
Poison — Poison Bolt • Poison Breath • Poison Cloud • Poison Spit • Poison Spray • Poison Sting • Poison Strike
Lightning — Lightning • Lightning Arrow • Lightning Globe • Lightning Storm • Red Lightning Hose • Shocking Hit • Spark Defense • Thunderbolt
Physical — Bite • Blast Wave • Blow Dart • Charge (Berserk Charge) • Cleave • Corpse Cannon • Destructive Strike • Divebomb • Grab • Ground Spikes • Ground Tentacle • Leap • Javelin • Leech-life Ball • Pummel •Quake • Quill Dart • Slam • Snatch • Spine Dart • Strike • Swipe • Sweep • Whirlwind
Wind — Cyclone • Gust • Tornado
Magic — Beam • Blade Beam • Blast • Bomb • Bone Spirit • Cataclysm • Hellish Projectile • Nova • Slow Time • Tide • Torrent • Unholy Bolt • Volley • Wave
Non-damaging Skills
Indirect Attack — Blood Mana • Curses • Mana Rift • Minion Frenzy • Putrify • Spin Web • Stun • Vortex
Spawning — Bone Prison • Festering Appendages • Lay Egg • Minefield • Mystifying Growth • Revive • Summon Minion • Totem
Self-effect — Bone Armor • Burrow • Effigy • Fly • Frenzy • Heal • Shell • Teleport • Vile Effigy