I’m planning on making some items in a game I plan on running, these items with involve losing something to gain something in return. Also they will be a way of an external force to try to corrupt a player if they hold onto it too long. Also thinking of possible way of evolving these items. This is my first item:

 
Long Sword (zweihander) 

Attack type - melee

Reach - 5ft

Damage - 1d12

Damage type - slashing

Weight - 12 lb

Cost -

Properties - two handed, sap

You gain a +1 bonus to attack rolls and damage made with this magical weapon.

Remorse 

Each morning role a d12 take that much necrotic damage (this damage can’t be healed, but can on a long rest). Put the number rolled to the side and can be save to a max of 24, this damage can be added to an attack but must be declared after hit and before damage is rolled. 

Anguish 

Twice per a long rest you can rolled a d12 on a creature you have hit add this damage (necrotic this damage can’t be healed, but can on a long rest) to that creature and you also take this damage. 

Well if it's common, most certainly, but if it's a higher rarity i'd say its not too busted

No this would be a one of six possible different items in the world and still thinking of the other five like armour or shield that prevents damage to a near by ally but you take either all or some of the damage, a staff of some sort and others. The idea is a great item but comes at a cost.

Just a suggestion: saying that each morning you take 1d12 necrotic damage that can't be healed adds some weird complexity to the rules around how healing works that might be more trouble than it's worth. You don't want to have to keep track of which damage came from which source.

Maybe consider making it a reduction to the wielder's maximum HP instead. That will prevent any annoying arguments about how healing works, and under 2024 rules changes to max HP are already reset on a long rest anyway, so you don't need to say anything else about that.

It is no more / less powerful than a regular +1 weapon IMO, personally I'd get rid of Remorse since that's just annoying to keep track of and keep Anguish.

You could add a Curse that forces the character to use Anguish in certain circumstances if you want it to feel more like it is corrupting them. For instance, you could add : "if you haven't killed a creature within the last 24 hours you must use Anguish on the next attack you make that hits."

If you want to tempt them to keep using it, then I'd add : "Each time you voluntarily use Anguish, roll a 1d20 on a 15 or higher this item permanently gains 1 additional use of Anguish per day". 

Its pretty cool, soo i think that it is decently justified, only if its really rare tho

I would say uncommon rarity, because common would be levels 1-5, which may kil the characters at that level. And it has already been said, but make it a reduction that actual damage. Apart from that, great!