Impregnable Mail is a fan made item set.

Fortified Shoulder Pad's Recipe

Impregnable Mail's Recipe

Unique Passive - Impregnable Armor

With Season 3 changes, percentage penetration is highly valued, if not borderline broken. When tanking I find myself melt in a second even though I have a full tank build. This item offers a counter to counter armor/mr stacking. The 20/35% reduction is calculated separately, if the enemy has 40% and 8 flat MPen it would become 26% and 5.2 flat with the mail.
The reason of % HP buff is when armor and MR fails, get more health, this item offers very little on it's own, you need and HP item and an armor or mr item to be effective, so it can't be a GA alternative for non tanks.
And the insane flat HP regen because I miss warmog's, FoN's and Randuin's.