The main difference between forgeries and genuine items is that forgeries will not have any magical properties the original did. So for example, forgeries of items like the Portcrystal and Ferrystone will not work. Some quest items can be forged, the originals being kept to benefit yourself somehow. Some items with special properties can be copied exactly with no problem - for example, Kept Sour Ambrosial Meat when forged will simply create more Kept Sour Ambrosial Meat. Rare materials can also be forged when needing to upgrade equipment.
A forgery will be mentioined as such in its name and description such as the Wyrmking's Ring Forgery. These type of items will differ in a way from the original, usually lacking some or all its magickal properties.
It is worth noting that many of the quests in New Game+ can be made easier by having the required items forged beforehand in your first playthrough.
If you are looking to complete the above achievement / obtain the above trophy, you must commission a forgery of a unique or magical item. Although typically expensive, this is the only way. Once you have retrieved your commissioned work from The Black Cat, it should read "(Forgery)" somewhere on the item, you should hear your ding not long after.
There is a chest, on a roof near The Black Cat, where you can find sometimes a forgery of a Wakestone shard.
Use of forgeries to change the outcomes of questsEdit
A Troublesome Tome - Giving Steffan Salomet's Grimoire will mean he aids you with magick in Griffin's Bane, later on. Give him a forgery and he will arrive to help you, but be unable to do anything. The original Salomet's Grimoire is a one-time use Bolide, so it's up to the player as to which will prove more useful.
Trial and Tribulations - two items can be found in Fournival Manor which are part of this quest, but may also be used to complete other quests. The items are the Gift Ledger, necessary for the quest Trail of Corruption and the Chamberlain's Affidavit, necessary for the quest Thanks Mislaid. A forgery of the items can be used to complete either quest.
A Lost Cause - The Snakeskin Purse is technically a material, so it helps to have a copy of it. The Bandit's Mask needs a Snakeskin Purse to be enhanced to three stars.
A Token of Esteem / Thick as Thieves - Forging Maul's Badge of Amity and Ophis' Badge of Amity will allow you to complete these quests whilst still keeping the badges.
The Wyrm Hunt Quests - After completing the Wyrm Hunt Quests, you will be asked to return your license. Considering it grants you a discount, it is recommended to make a copy before handing it over.
Escort Duty /Idol Worship - The Gold Idol acquired from Escort Duty can be forged. Giving a forgery to Caxton or Madeleine should NOT be done, as it will not gain you anything; instead give one of them the real idol, then give the Silver idol to the other for the best results. You should forge the Gold Idol because the forgery will still grant you a discount.
The Conspirators - Upon completing the objectives of this quest and returning to the castle, you will be offered a choice of taking ths letter to Fedel or giving it to Ser Gabrian. Forge the letter and you can give it to both, reaping double the rewards.
The Wyrmking's Ring - The Wyrmking's Ring reduces spellcasting times slightly, so it can be beneficial to keep it for yourself. It can be forged, and then the forgery given to Aldous. Later, the Duke will take you to the Treasury to open a chest with the Paladin's Mantle inside. If you gave Aldous the original, it will open, if you submitted the forgery, the Duke will assume it has lost its magic, and you will be unable to receive it. Once you have the Wyrmking's Ring it's possible to sneak in to the castle at night, wait for the soldier guarding the treasury to walk outside, then go into the treasury and take the Paladin's Mantle for yourself (a quick escape with a Ferrystone is recommended), so there's little reason not to submit a forgery.