The item is 100% gold efficient when the champion has 1204 maximum health.
All champions have between 1260-1764 base health at level 13, therefore, this item is cost efficient for every champion in the game by level 13.
Atma's Impaler adds directly to attack damage. It is not an "on-hit" effect. .
The unique passive grants the same amount of attack damage as a B. F. Sword (50) at 3333 health.
Champions with large health pools or health-gaining abilities, like Sion and Cho'Gath, profit greatly from Atma's Impaler.
Atma's Impaler is one of the most cost-effective items in game, surpassing Pickaxe on any level 18 AD champion (without HP items).
Along with being built from a gold-gen item (Avarice Blade), the cost-effectiveness is increased.
Atma's Impaler can be combined with items such as Frozen Mallet and Warmog's Armor to utilize both damage and survivability. These builds are known in the community as "Fratma's" and "Atmog's", respectively, "Fratmog's" for both at the same time.
Atma's Impaler is one of only a handful of items beneficial to an attack-based character that provides armor, the only others being The Lightbringer, which along with not being available on SR, provides less: armor, late-game AD and no crit chance; and Iceborn Gauntlet which (while not giving any direct stat boost to AD) provides bonus damage on-hit damage on basic attacks and provides more armor at a higher cost.
However, some champions can use this armor in synergy with AD magic resist items for a balanced defense, and making use of the Atma's Impaler's critical chance to boost items like Infinity Edge.
The weapon's name is a reference to the Ultima Weapon (originally called "Atma Weapon") from Final Fantasy VI and subsequent games, a sword which deals damage according to the hero's health.