I’m a new DM with all new players, so I’m slowly working through introducing as many aspects of the game as I can in our first campaign and I think it’s time for a cursed item. The party accidentally stole a box containing a magical item from a cult, they haven’t managed to open the box yet so I don’t have to know what the item is until they do. I’m trying to achieve a balance where the cursed aspect would be considered a negative to good aligned creatures, but not necessarily for evil aligned creatures. I have two ideas and I would love to hear feedback on which you think might make for more interesting gameplay/discovery if the curse. 

Dagger of Tracking: +1 to attack and dmg; the dagger will have a slight physical pull in the direction of the last creature to take damage from it, as long as it has more than 0hp; requires attunement. 

Option 1: The bearer cannot rest until the dagger’s target is slain- taking 1 point if exhaustion every 24hrs. 
Option 2: It the target is killed by anything except the dagger, the bearer must succeed a CON save or take the same damage as the killing blow. 

In the hands of a cult/assassin these curses shouldn’t be a problem, but if the party ever tries to show mercy in a fight (they’re very not murder hobos) it would be a problem. 

Option 1 - optional rules allow for a CON save for the PC to avoid the level of exhaustion. I'm not suggesting that it be required, only that it be considered. In the long run, if someone is injured by the dagger, and flees the PCs influence, the PC is in jeopardy of death by exhaustion.  If this line is followed, the PC only has two choices, kill everything they cut, or find a way to remove the curse. I'm assuming that the method of detecting the curse is through experiencing the side effects?

Option 2 - close to the same as Opt. 1, only potentially more lethal to our PC in niche circumstances. If our cursed PC were within a few points of unconscious and someone nukes the PC's last target, there is a potential (level and class dependent of course) that the PC could be killed outright.  Of course this is rather hyperbolic and a very niche possibility, but still a possibility given that the PCs that would traditionally use a dagger don't stand out as high CON, high HP stereotypes.

IMO, curses should be put into play to hinder, not kill. I try to avoid the curse being used as a way to tailor PC actions for the long term, or to eliminate choices that they would normally be allowed.  Strong effect curses might be used in the immediate term, but I avoid allowing them to have a lasting effect on the party or a PC.

I have to agree with Kaavel. Unless your players really enjoy a gotcha kick in the teeth, I would avoid a curse that forces a player to entirely change their playstyle or that has such an easy outcome of dead PC. For new players who may very well be unaware cursed items are even a thing I like to introduce something that has almost no downside keeping in mind that attuning to a cursed item already comes with the negative of losing an attunement slot until they can break free of the item.

Some other options I may suggest 

- dagger gives a slight pull towards last hit target as long as it has more than 0 hp and give them disadvantage if they attack a different target. Have the effect wear off at the end of their next turn. Makes it hard for the target to hide from player and maintains that they get locked into a target but does not remove player choice. I would still consider this a little harsh depending on your players but I know I have some players that would enjoy this.

- make it very simple and it is just a dagger +1 that lets them briefly track their last hit target. If they crit miss an attack they hit themselves instead.

Ziek88 comes up with an interesting option in a simple dagger +1.

To the player it is a dagger+1, but to you (the DM) it is a dagger-2. Effectively making it -1 to hit and damage. Much like the Stone of Ill Luck. Small hinderance that the player will feel tugging at their meta knowledge but unable to fully put their finger on.

For early cursed items, it's better for the curse to be a minor inconvenience than one which could potentially send the game off on a curse-removal tangent.

I like the flavour of the magical effect, perhaps the curse is that the dagger stops working for as long as something it has damaged is still alive? So if you attack 2 different targets, the dagger stops working. If you attacked Jim the actually-friendly goblin, the dagger loses its tracking abilities until Jim is dead (IE, only every tracks Jim).

Curses which affect the person using it in actually adverse ways (IE, not just a quirk of the magic) I reserve for rare+ items; uncommon ones I stick to inconvenient curses!

Well as much as I want to make something truly evil, I’ll take the advice and save that for a bit later in our campaign! I’ve decided on making the pull of the dagger get stronger and stronger the longer it tracks a single target, the user will have to make strength saves with an increasing DC to stop the dagger from launching itself towards the tracked target. Once per day or two, at inopportune moments, hopefully not while in a crowded inn or something if that sort. ;)

Agree with what is said earlier, killing your players sucks, especially if it's something they had no way of knowing about or playing around.

As a side note, for your first cursed item, it can be better to troll your players instead. Give them a ring of invisibility, but say it looks very old and somewhat damaged. When they try to use it, tell them nothing happens. Until they go to take it off. Then let them know, oh it turned invisible. Prepare to have something thrown at you.

I recommend giving normal, everyday items curses. Not like 4 out of the 5 mugs on the table are cursed, but more of an odd inconvenience. This keeps your players on their toes whenever they take treasure. Over time, multiple minor curses can be very detrimental, if not deadly. Keep the major, game-changing curses on rarer items, and make curses an uncommon mishap, and you'll guarantee that the party spellcasters always have detect magic prepared.