The Ancient unit tag causes the tagged units to be treated differently by spells and other effects. It also turns them completely immune (i.e. untargetable) for some spells, mainly spells which instantly kill the target.
The following units are ancient creeps: Ancient Black Dragon, Ancient Black Drake, Ancient Granite Golem, Ancient Rock Golem, Ancient Thunderhide, Ancient Rumblehide, Ancient Prowler Shaman, Ancient Prowler Acolyte, Roshan, Animal Courier, Flying Courier, Earth, Storm, Fire, 
Astral Spirit, Familiar, Lua error in Module:VarQuery at line 21: No results found.. Warlock's Golem, 
Tombstone.
Spell interactions
If a spell has different effects on heroes as it does on creeps, then it uses the creep effect on ancient creeps. For example, spells with different durations on creeps than on heroes use the creep duration.
Not affecting
The following spells are unable to target or affect units tagged as ancients.
Bane – Enfeeble- Clockwerk – Power Cogs
- Dark Seer – Vacuum
- Doom – Doom
- Elder Titan – Natural Order
- Elder Titan – Earth Splitter
- Enchantress – Nature's Attendants
- Lua error: Found no Cargo data for "Enigma - Demonic Conversion".
- Enigma – Midnight Pulse
- Hand of Midas – Transmute
- Helm of the Dominator – Dominate
- Keeper of the Light – Blinding Light
- Nature's Prophet – Sprout
- Necrophos – Heartstopper Aura
- Lua error: Found no Cargo data for "Outworld Devourer - Arcane Orb".
- Pangolier – Rolling Thunder
- Pudge – Meat Hook
- Radiance – Burn
- Lua error: Found no Cargo data for "Shadow Demon - Soul Catcher".
- Spirit Bear – Entangling Claws
- Vengeful Spirit – Nether Swap
The following spells cannot target units tagged as ancients by default, but can be made to target them with upgrades.
1 These spells can target or affect ancients when having Aghanim's Scepter.
2 These spells can target or affect ancients after the required 
talent is chosen.
Note: If the target is a creep-hero with the ancient unit tag, these spells still treat them as heroes, Holy Persuasion and Devour still cannot target them and Enchant slows them, rather than converting.
Unique
The following spells treat units tagged as ancients differently than regular creeps.
-
Uses the hero duration against ancient creeps, rather than the creep duration.
-
Does not root enemy ancient creeps like how it does when used on regular creeps.
-
Cannot be cast on ancient creeps to smash them. Ancient creeps are affected by the damage/slow normally.
-
Cannot be cast on ancient creeps. Can jump onto ancient creeps and damage them normally.
-
Cannot be cast on ancient creeps when on level 1. Can target them normally when on level 2 or 3.
-
Does not instantly kill ancient creeps like how it does when hitting regular creeps.
-
Does not affect/damage enemy ancient creeps. Does affect/heal allied ancient creeps.
-
Does not affect ancient creeps when passing through them. Does affect ancients while stationary.
- Timbersaw - [[Timbersaw#|]]Does not affect ancient creeps when passing through them. Does affect ancients while stationary.
-
Cannot toss ancient creeps. Can target ancient creeps to toss units towards them.
