How?

Look in the items folder and find "sets.txt" and dig through a bit more to find "level_16_unique.txt"

From "sets.txt":

"[set]
id=4
name=Underworld Armament
color=255,64,255
items=421,422,423,424,425,426,427,428,429
bonus=3,currency_find,25
bonus=4,speed,125
bonus=5,xp_gain,15
bonus=6,absorb_min,20
bonus=6,absorb_max,20
bonus=7,fire_resist,40
bonus=7,ice_resist,40
bonus=7,lightning_resist,40
bonus=7,dark_resist,40"

From "level_16_unique.txt":

"# Stronghold #1
[item]
id=421
name=Underworld Cloak
INCLUDE items/base/armor/mage_alt2/head.txt
level=16
quality=unique
price=960
abs=6,7
requires_stat=defense,27
requires_stat=mental,16
bonus=mp,54

[item]
id=422
name=Underworld Platemail
INCLUDE items/base/armor/plate/chest.txt
level=16
quality=unique
price=2400
abs=8,9
requires_stat=defense,28
requires_stat=physical,14
bonus=hp,600

[item]
id=423
name=Underworld Boots
INCLUDE items/base/armor/plate/feet.txt
level=16
quality=unique
price=960
abs=7
requires_stat=defense,27
requires_stat=offense,15
bonus=avoidance,48

[item]
id=424
name=Underworld Armguards
INCLUDE items/base/armor/plate/hands.txt
level=16
quality=unique
price=800
abs=7
requires_stat=defense,27
requires_stat=offense,15
bonus=accuracy,48

[item]
id=425
name=Underworld Greaves
INCLUDE items/base/armor/plate/legs.txt
level=16
quality=unique
price=1600
abs=8
requires_stat=defense,28
requires_stat=physical,14
bonus=poise,48

[item]
id=426
name=Underworld Tome
INCLUDE items/base/misc/spellbook.txt
level=16
quality=unique
price=1600
requires_stat=defense,26
requires_stat=mental,16
bonus=mp_regen,48

[item]
id=427
name=Underworld Blade
INCLUDE items/base/weapons/melee/greatsword.txt
level=16
quality=unique
price=3200
dmg=melee,85,90
requires_stat=physical,30
bonus=dmg_melee_min,30
bonus=dmg_melee_max,30

[item]
id=428
name=Underworld Bow
INCLUDE items/base/weapons/ranged/greatbow.txt
level=16
quality=unique
price=3200
dmg=ranged,85,90
requires_stat=offense,30
bonus=dmg_ranged_min,30
bonus=dmg_ranged_max,30

[item]
id=429
name=Underworld Sceptre
INCLUDE items/base/weapons/magic/greatstaff.txt
level=16
quality=unique
price=3200
dmg=ment,85,90
requires_stat=mental,30
bonus=dmg_ment_min,30
bonus=dmg_ment_max,30"

It looks good?

There are 6 items which everyone can wear:  Helmet, Armor, Boots, Gloves, Leg-Protection and Artifact.

You must chose 1 of the 3 weapons but this is enough for 7 in total.