I just found one today in the edge of a darkwood biome. Currently there's a few unique mobs there, a helmet crab, armored goblin (which swings a giant flail which knocks you 2-3 blocks into the air upon collision with the player) and a bigger armored goblin (which is two goblins in one big suit of armor) The bigger armored goblin carries a shield which absorbs damage and lasts for several hits. I find using critical jump-attacks often smashes the shield. The mob also carries a spear with a fair bit of reach and is used to perform a downward attack, usually when drops are present on the ground, which will destroy those drops.
As mentioned the bigger armored goblin is two seperate goblins, one is standing on the other's shoulders, wearing a single suit of armor. Attacking the lower half until killed will leave the upper half and vice versa. It is preferable to kill the upper half before the lower due to the drop-destroying spear attack.
Currently they drop armor shards, which can be combined in a 3 x 3 pattern into a shard cluster which is smelted into a knightly ingot. 33 Knightly ingots can make an armor set, a knightly pick, axe and sword. I'm not sure what the benefits are compared to other armor however the tools provide a boost against "Armored targets."
They look cool though.
Armored mobs absorb 75% of damage taken by normal melee attacks, hence the natural buff of the Knightly Tools.
Knolls and Goblin Miners also spawn inside and yield drops and exp as normal.
They also have crypts and graves which have chests underneath. Some chests are actually Trap Chests which have TNT underneath. Chests sometimes yield knightly shards and ingots among other remedial Twilight Forest chest loot such as charms of keeping.
The boss room currently has about 6 or 7 ghost-like figures which fly around in circles around you, have 70 HP each-ish, don't attack you and currently only drop EXP. There's a sign in the boss room saying "Boss Room Out of Order".
The layout of the dungeon is much like a multi-floored stronghold and uses a block called "Stronghold Shield" which surrounds the outer walls of the stronghold and acts as bedrock. The walls themselves of the stronghold are made of normal, mossy and cracked brick, similarly to normal strongholds. There are red and green sides to the stronghold shield, the green side can be mined but yields no drops. The blocks, as far as I'm aware, cannot be destroyed by TNT.
