History of Battle-Forged

Battle-Forged is a triumph: complete 3 Battlegrounds in Fireteam Ops. Reward: 20 Legendary Marks. It sits in the Teamwork: Strikes and Battlegrounds section of the Monument of Triumph list, and no seal was listed for it (ProGameTalk).

How to complete

1. Open the Director and choose Fireteam Ops, then pick a Battleground (three-player activities scored by enemy kills). Matchmaking is available at Expert difficulty and above; Normal can be run with your own fireteam.
2. Complete any 3 Battlegrounds. Current rotation, per Shattered Vault, includes Heist Battleground: Mars, Behemoth, Echo: Core, Heist Battleground: Moon and Defiant Battleground: EDZ.

Tips

Lower difficulty is fine for the triumph, since only completion counts. The sources do not say whether the three must be different Battlegrounds or done in one sitting.