Updated 08-27-2016 06:01 PM
This achievement is unlocked by accumulating 60,000 Guild Marks. These Guild Marks can only be acquired if you are part of a guild. Once you are in a guild you can travel the guild's Stronghold via the travel menu. Once you arrive in the Stronghold there will be NPCs that give you quests withing the zone that will reward you with items that you can donate to the stronghold coffer. The coffer looks like a Mimic (A large Treasure Chest with a tongue sticking out of it) is located in the back left corner of the Stronghold.
The purpose of the coffer is to upgrade and build structures within the zone to gain boons that will benefit your character/guild. You can donate most items found in the game to the coffer. The guildmaster will set a target structure to work towards which can be seen by clicking on the Mimic. Donated items have will vary in Guild Mark value. You can also hoard items and wait for a double Guild Mark event to gain twice as many marks. There is a Guild Mark cap of 30,000, but if your Stronghold(or Alliance) has built a marketplace then you can spend the Guild Marks there which frees you up to then donate more items to work towards the 60,000. You can also gain the Guild Marks on two different characters to get to the 60,000 (30,000 on each character) and the achievement to pop. The best armor for your character called Dragonflight can only be attained with these Guild Marks. Here is a list of some of the items you can donate to the coffer for the Guild Marks:
Resources (From Zone Quests)
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Heroic Shards of Power
Dungeoneer's Shards of Power
Heroic Shards of Power
Influence
Stockpiles
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Astral Diamonds & AD Chests
Gold
Gems in the form of Enchantments or Runestones
Glory earned through PVP matches
Labor in the form of crafting professionals
Surplus Gear, this is any identified gear you find and don't need
Treasures (Campaign Quest Items)
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Treasures of Tyranny in the form of Dragon Hoard Coins, Pages of Arcane Lore, Cult Secrets, and other items from the Dragon Campaigns
Fey Trinkets in the form of Gold Crescents, Feywild Sparks, and other items from the Sharandar Campaign
Dark Gifts in the form of Thayan Scrolls, Vangard Scripts, and other items from the Dread Ring Campaign
Frozen Treasures in the form of Auril's Tears, Dwarf Gold, Konig Coins and other items from Icewind Dale Campaign