Pact of The Hordmaster

Your patron is a powerful being that has amassed huge amounts of wealth, knowledge, and power, and is willing to use these resources, and you, in order to gain more. The only goals of this creature are to gain more power, or receive more treasure, and they often are willing to do just about anything to get more of it. Because their power comes from this great horde, they are willing to grant you a part of it in order to preform their work, but are more willing to give you power in exchange for valuables to grow their horde even larger. Creatures that could qualify as Hordemasters could be such things such as very powerful ancient dragons, liches, kraken, or any other old, powerful, and greedy creature who is capable of giving you power in exchange for expanding its treasures.

Heart of Greed

As an action on your turn you can attempt to pull on the greed in the heart of one creature within a 30 feet of you. The target must make a wisdom saving throw vs. your warlock spell save DC. On a failed save the creature becomes charmed by you for one minute. While under the effects of this charm you have advantage on all rolls made to persuade the charmed creature to make any action that involves the exchange of valuables, but not on any other ability checks. After this effect wears off they feel as though all actions done during this time were of their own volition, and thus know nothing of your involvement.

Gifts of the Hoarder

Your patron grant you the knowledge of exactly how much an object is worth. When you touch an object you can learn it's exact value in gold pieces. You also gain access to a pocket dimension where you can store valuables. You can place only non-magical valuables such as coins, gems, art pieces, or other valuables here, but you may store as many applicable items as you wish. When you die the contents of this dimension are added to your patron's horde

Rate of Exchange

At 6th level, your patron grants you the ability to exchange spell slots of gold, and vice versa. As a bonus action on your turn you can exchange 100 gold pieces per spell level to regain a spell slot of that level. You can also expend a spell slot to receive gold pieces 50 gold pieces per level of the spell. These gold pieces appear in and are taken directly from the storage space provided by your gift of the hoarder feature. This feature cannot be used to create spell slots of a higher level than you can cast using your Pact Magic feature.

Open Market

At 10th level, you may use your connection with your patron to buy objects from his hoard, or sell them to it. You can meditate for 10 minutes to communicate with your patron and organize the terms of the trade. The Hordemaster, however, always takes a cut of the trade, and the item must be something that could reasonably be in the Hordemaster's hoard. For example if the characters are after a Holy Avenger, you cannot simply buy it using this ability, unless it sits in the lair of your Hordemaster. See the rate of exchange table to determine the multiplier, that when multiplied by the starting price determines how much an object can be bought or sold for using this ability. You may use the middle of the price range suggested in the PHB, or DMG for magical items, when determining starting price, or the DM can set a starting price based on the rarity of the item. These gold pieces appear in and are taken directly from the storage space provided by this subclass. The prices for legendary and artifact rarity items are set by the Dungeon Master based on how important these items would be to the patron.

Rarity               | Sell   | Buy
Non-magical | .3 x starting price    | starting price
Common        | 0.5 x starting price | 1.1 x starting price
Uncommon   | 0.7 x starting price | 1.3 x starting price
Rare                | 0.9 x starting price | 1.5 x starting price
Very Rare       | starting price          | 1.7 x starting price
Legendary     | D.M.                          | D.M.
Artifact          | D.M.                          | D.M.

Re-balance the Scales

At 14th level you gain the power to alter the power of spells in an instant, transferring this power to your patron, or buying it from him. At any time when a spell is cast you may use your Rate of Exchange feature to purchase a spell slot of any number. You may boost the level of triggering spell by that many levels. These gold pieces appear in and are taken directly from the storage space provided by this subclass.
Alternatively, may also use this feature to lower the level of a spell cast. The caster must succeed on a saving through using the spellcasting ability of the triggering spell. On a failure the spell's level is reduced by a number of levels of your choice. This must drop the spell to it's base level, or zero which causes the spell to fail. You then gain the same amount of gold in the storage space provided by this subclass as if you were to sell a spell slot of the selected level.
You may only use this feature to raise or lower a number of spell levels per long rest equal to half your level. For example a level 20 pact of the Hordemaster warlock would be capable of reducing a single 9th level spell slot to nothing, and then boosting a single ally's spell by one level later in the day, or the same character could boost his own spells twice by five levels each. This ability can cause spells to rise above 9th level.