From Halopedia, the Halo wiki
The IWHBYD Skull
John-117 tracked the UNSC signal to a Banished facility known as the Tower. Its warden, Chak 'Lok, denied the Spartan access into the Tower by putting the outpost on lockdown. With guidance from the Weapon, John-117 lifted the lockdown, and entered the Tower via the gravity lift at its base. Inside, John-117 discovered Spartan Hudson Griffin being restrained and tortured. Although Chak 'Lok intervened, he was killed in battle against John-117. Upon being freed, Griffin told John-117 of the Banished's search for the Conservatory at the excavation site north of the Tower, but passed away shortly.
Once you have arrived at the entrance to The Tower
you will need to defeat all the Grunts and Brutes you will need to go to the gatehouse near the gate and open the gate if you make your way toward the Gravity Lift it will shut off and you will need to go to the Security Building
which is near the gate you came in through. Up the stairs in the building there is a console which disables the lockdown. After you will need to make your way to the switch on top of a platform near the lift, once you activate it you will need to fend off Banished reinforcements.
When you get up the gravity lift you will investigate a suit of MJOLNIR Powered Assault Armor (GEN3) belonging to Spartan Hudson Griffin. You will then have to make your way up 3 floors filled with Banished.
Once you have got to the top floor you will find a door leading to a room with the Spartan in it once you enter you will trigger a cutscene where immediately afterwards you will be in combat with Chak 'Lok where he has Active Camoflague and you will need to use your Threat Sensor to find him. Afterwards you can leave through the door at the back of the room where you can cross a newly extended bridge to get to the other part of the map.
Hudson Griffin's MJOLNIR (GEN3) Armour