Iron Temple is a Grimoire card from the original Destiny (Destiny 1), found under Places → Earth. Grimoire cards held most of the game's lore and were read on Bungie.net and in the companion app, not in the game itself.
Card text
...And wherever we Iron Lords stake a claim worthy of battle, let us immortalize our victory with a temple, a beacon for all who walk the waste to follow. And let the truths of that battle live only within its crypts, kept sacred by an everlasting oath worthy of our Light and sacrifice, so that our peace will never need know the horrors it faced, sealed in secrecy until the Light dims to Darkness, forevermore....
Grimoire text written by Bungie, shown as it appears in the Destiny 1 game files.