Something in the lines of mentem could work to make the attack forgotten. About pain and penalties, there are really a problem... unless there is no actual wound making the pain.
So I'd go with the following effects:
Forgotten Stab
PeMe 20
Anyone attacked with the dagger forgets that he was attacked with it. This leaves a blank spot on the memories of the victim.
Base 4, +1 Touch, +1 Diameter, +10 Unlimited uses
Gentle Assassin
CrMe 38
This effect is triggered by The Forgotten Stab effect and fills the memory of the victim with the memory of them talking to the attacker about trivial and boring stuff.
Base 5, +1 Touch, +3 Moon, +10 Unlimited uses, +3 Linked Trigger.
Delayed Death
CrCo 63
Any up to lethal wounds inflicted by the dagger are instantly closed while the dagger kept concentration. The dagger stops doing so when someone presses his lips over the blade and whispers in latin "dust to dust".
Base 35, +1 Touch, +1 Concentration, +5 Item maintains concentration, +10 Unlimited uses, +3 Linked Trigger.
I'm not sure about the linked triggers though... maybe they wouldn't be required at all, specially the one in Gentle Assassin, but adding it could make it fill the gap created by the first effect, making the effects cascade in the memory of the victim. The last one in Delayed Dead would be mainly to keep duration flexible, but maybe it's easier to make an instilled effect stop when desired by default. Anyhow the last effect have a quite big level. So the problem at the end is that it's quite more easy to enchant a dagger just to kill someone with one single effect: it can be done at any time.
Distant Stab through the Heart
PeCo 50
If the dagger stabs an arcane connection to the victim (it could be a puppet washed in the victim's blood, or covered with a lock of his hair, or clothed with some of the victim's usual clothing), then a fatal wound the size of the dagger is opened right from the chest and through the heart of the victim.
Base 30, +4 Arcane.
Or you could lower it all by making the item being able to open an intangible tunnel to the victim, then maybe seal it while maintaining concentration until the desired time on the attack, and then use a level 35 version R: Touch of the above effect through the tunnel.