The Collector

An elusive being with a hobby more sinister than gathering harmless trinkets, The Collector is an Eldritch/Human miniboss who has a 5% chance to spawn if your inventory is over 65% full. It carries with it the Puzzling Trapezohedron, a gem worth 2500 gold, or one of three unique trinkets: Barristan's Head, Dismas's Head, or Junia's Head.

Strategy
The Collector doesn't have the highest of stun resistances or speed, you should have a chance to stun him on first round before he uses Collect Call and damage him significantly.

Once Collect Call is used, target any Collected Highwaymen first, as he deals high damage and can inflict bleed. Stun Collected Man-at-Arms to break his guard on Collector or Highwayman, ignore Collected Vestal. The ideal composition to deal with is 2 Vestals and Collector, Vestals dont have offensive abilities, you can outdamage their heals and sometimes they will waste their turn on buffing, from rank 3 Collector usually uses his weakest move, Show Collection.

it should be considered the possibility of moving him to the first 2 front ranks as from that position he will be unable to use his "Life Steal", thereby accelerating the process of killing him while also reducing his damage output significantly.

After the fight you can spend a few turns healing, Collected Vestals and Man-at-Arms are harmless.

Behaviour
COLLECT CALL: the more enemies that are on the field, the less likely the chance is for the Collector to use his "Collect Call". More specifically, the numbers are 1000x if no enemies at all (meaning he will virtually always open with these move), 9x and 3x for 2 and 3 enemies respectively. He will not use this skill if all ranks are covered.

SHOW COLLECTION: 3x chance for the Collector to use this skill.

LIFE STEAL: 2x chance of him attacking your heroes with this ability.

Note that there are no 1x numbers, that means, for instance, that should all ranks be occupied, 2 out of 5 times he will use "Life Steal" and the other 3/5 "Show Collection". If there are 3 enemies the probability should read: 2/9 for "Life Steal" and 1/3 for both "Show Collection" and "Collect Call". For 2 enemies, 1/7 will roll on a "Life Steal", 3/14 on a "Show Collection" and 9/14 on "Collect Call". If the Collector is alone, it is safe to assume he will be using "Collect Call".

Abilities
* The possible summons are the Collected Highwayman, the Collected Vestal and the Collected Man-at-Arms ** While none of The Collector's attacks or those of his summoned heads deal extra damage against marked targets, it does allow the "Collected Highwayman" to focus his attacks on marked heroes. *** The Collector heals for the damage inflicted plus a small amount. It seems to be +5-10, most likely increasing with the difficulty of the dungeon. This needs confirmation, however.

Trivia

 * The Collector is a reference to Hastur, otherwise known as The King in Yellow.
 * The cage that the Collector wears was once a common form of torture employed for prisoners and lunatics. Spikes could be outfitted to the bottom of the cage to inflict additional pain on a subject being tortured should they attempt to crane their heads in response to pain. Additionally, some models featured sharpened spikes or spurs that projected from the horizontal cross-bars of the cage into the mouth, whereupon they dug into the tongue if the victim attempted to speak and were often fitted onto those accused of heresy or witchcraft. A variant called a "scold's bridal" or "the branks" was even employed to silence and humiliate housewives between the 1500-1700's by means of a curb plate which projected into mouth and pressed down upon the top of the tongue, preventing speech.
 * One of the trinkets dropped by the collector is called Dismas's head, so it can be inferred that it is the same head as your starting highwayman is called Dismas. However, you can find this trinket while Dismas is still alive.