Warmog's Armor is a legendary item in League of Legends. 
Cost Analysis *
* is 105. gold efficient, without its passive.
26% With its passive, it's at least 640% gold efficient. For every 100 health, it becomes another 18% more efficient. (For every 5.55 health, it becomes 1% more efficient)
The living armor protected the greatest troll warrior in the entire realm during the bloodiest and most devastating battles of the Rune Wars. Deep within the dark woods of Crystone the living armor waits to protect its next owner. A word of warning required before seeking out Warmog... it will protect you for a time, but when it grows tired of you, who will protect you from it?
The previous unique passive used to belong to the removed item .
Strangely, the nexus obelisk (spawn turret) does not stop the passive. This may be due to the unique type of damage that the tower "deals", "pure damage".
New Recipe: + + + 400 = 2850
Old Recipe: + + + 550 = 2850
Health reduced to 800 from 850.
Now grants 10% cooldown reduction.
Total cost increased to 2850 from 2750 .
Health increased to 850 from 800.
Now grants 200% base health regeneration.
New Effect: You gain Warmog's Heart once your maximum health exceeds 3000.
New Passive - Warmog's Heart: If you have not taken damage in the last 8 seconds, you will restore 3% of your maximum health every second.
Item cost increased to 2750 from 2500 .
Combine cost increased to 550 from 300 .
Available on Howling Abyss.
Recipe changed: + + + 300
Item cost reduced to 2500 from 2830
Health reduced to 800 from 1000
Unique passive - Restores 1% of maximum health every 5 seconds. Health restore increases to 3% of maximum health if damage hasn't been taken within 8 seconds.
Combine cost increased to 1070 from 995 .
is no longer available in Howling Abyss (The Proving Grounds).
Passive Health Regen reduced to 1% of maximum Health per 5 seconds from 1. 5%.
Recipe changed: + + + + 995
Total cost increased to 2830 from 2650 .
Recipe: + +
Item cost: 2650
Combine cost: 995
NEW Unique Passive: Restores 1. 5% of your maximum health every 5 seconds.
This item no longer gains health and health regen per unit kill.
December 21, 2011 Hotfix
Fixed a bug where would go beyond 350 bonus health on assists.
Passive health gain cap reduced to 350 from 450.
Passive health regeneration per 5 seconds reduced to 10 from 15.
Health gain per kill or assist reduced to 35 from 45, minion kill reduced to 3. 5 from 4. 5.
Health regeneration per 5 seconds gain per kill or assist reduced to 1 from 1. 5, minion kill reduced to 0. 1 from 0. 15.
No longer purchasable in the Dominion game mode.
Tooltip will now show the current stats on its passive.
Combine cost reduced to 980 from 1100 .
Health increased to 920 from 770.
Passive changed to: Permanently gain 4. 5 health and 0. 15 health regeneration per 5 seconds per minion kill. Champion kills and assists grant 45 health and 1. 5 health regeneration per 5 seconds. Bonuses cap at +450 health, and +15 health Regen per 5.
Health reduced to 850 from 900.
Recipe cost increased to 1100 from 1075 .
May 15, 2009 Patch
Maximum health stack increased to 500 from 400.
Health regeneration per 5 seconds cap increased to 62. 5 from 50.
April 18, 2009 Patch
Recipe cost increased to 1025 from 765
Total cost increased to 3000 from 2740
Added a cap for bonus health / regen of +400 Health, and 50 Health regeneration per 5.
Health gain per kill modified to 4 from 5 (to 40 from 25 on champions), and regeneration per kill modified to 0. 1 from 0. 05 (to 1 from 0. 5 for champions).