The ritual dagger is a special dagger that is used to perform the ritual to strip demonic powers. Parker Caine somehow came upon the dagger, before it was passed on to Abigael Jameson-Caine.
History
At some point after leaving to find himself in "The Source Awakens", Parker Caine came into possession of the dagger. He attempted to use the ritual in "Past is Present" to strip his half-sister, Abigael Jameson-Caine, of her demonic powers, but she was able to retain them due to having eaten an apple from the hallowed tree in Sacred Grove.
In "Needs to Know", Abigael is approached by Macy Vaughn who wants help in removing her demonic powers - in hopes that this will aid in restoring the Power of Three. She escorts Macy down to the dungeon, revealing that Parker is alive as he is the only being that knows where the ritual dagger is that is needed. Abigael threatens Parker, even telekinetically flinging him up against the ceiling of his cell, before Macy says that she will speak with him.
Later, after having obtained the knife, Abigael performs the demonic ritual to strip Macy of her demon powers.
In "Sudden Death", Maggie Vera was sure that Abigael had used the dagger to do something to Macy and prevent the Charmed Ones from recovering the Power of Three. However, to be sure, she needed the dagger to perform a provenance spell. So, while under the pretense of asking Abigael for help in dispelling Jordan Chase's family curse, Maggie used an influencer spell on Abigael's henchwoman to take her to where the dagger was kept - the same dungeon where Parker was imprisoned.
Parker wanted to talk to Maggie, but Maggie just wanted to find the dagger, which Parker stated was hidden behind a pipe in a secret compartment. Unfortunately, Abigael caught on to what Maggie had planned, and snatched the dagger from Maggie, believing she wanted to use it to strip Parker of his powers so they could be together. Maggie explained her belief that Abigael had done something to block the Power of Three, a statement Abigael found amusing, since, as she gladly explained, she didn't do anything of the sort. She willingly allowed Maggie to perform her spell, but demanded she return the dagger once finished, and to then leave.