(10) יהוה spoke to Moses, saying, (11) “Phinehas, son of Eleazar son of Aaron the priest, has turned back My wrath from the Israelites by displaying among them his passion for Me, so that I did not wipe out the Israelite people in My passion. (12) Say, therefore, ‘I grant him My pact of peace.
Here is something we must understand. It is known that HaShem deals with humanity measure for measure, what we put out, we get back. That being the case, and we see Pinchas had a reward of receiving a covenant of peace with HaShem based on his zealousness for his Lord.. so what was the act that he did that actually caused him in a karma sense to receive this pact of peace?
And we can preface with what was taught in Agra D'Pirka -
"It was told to me by the holy mouth of the Holy Rav, our master and Rabbi, Shlomo of Karlin, may his soul have an everlasting memory, and may his blood be avenged, who taught this over in the name of the Medrash - At the time when HaShem told Elijah the Prophet that he would be assigned to attend the circumcisions of all the people of Israel, for all time.. Elijah responded back to Hashem saying "You know that I am zealous for your blessed name Hashem, and if the father of the baby boy is a sinner, I wont tolerate being able to attend" So HaShem guaranteed Elijah that the father of the baby will be forgiven and absolved of his sins. Elijah said back to HaShem - "Perhaps the one doing the circumcision is a sinner as well", so Hashem said he would forgive him as well. Elijah again said back to HaShem - "Perhaps some of the congregants joining will be sinners, and I wont be able to tolerate being there" So HaShem promised him that He will forgive the entire congregation. And these are the words I heard from his holy mouth"
And it seems to me, that we know that Pinchas is the same soul as Elijah, as we can see below.
לאו כהן מר - דאיכא למ"ד דאליהו הוא פינחס דכתיב ביה קנא קנאתי וכן בפינחס בקנאו את קנאתי:
There are those that teach that Elijah the prophet is Pinchas in reincarnate, as we can see from the verses the embedded hint regarding their equal zealousness
They were both zealous for HaShem and brought atonement for the people of Israel. So because of his attribute of being zealous, and his role at being at every circumcision, it forces HaShems hand to forgive the children of Israel, as Hashem must keep to His assurances.
Therefor, we see that the power of Elijah the Prophet is that wherever he goes, he brings atonement for the people of Israel, and with this we can understand that when the time of the redemption comes, HaShem plans on sending us Elijah the Prophet, remembered for good, prior to the redemption, as we see in the verse -
Since Elijah the Prophet will be coming to Israel, HaShem will immediately forgive the sins of all of Israel, and Israel will be pure and clean from any semblance of a sin or infraction. And through this, we will be worthy for the final redemption.
And now we can understand well the reward that HaShem gave him, that since Elijah the Prophet made peace between the children of Israel and HaShem, via HaShem forgiving their sins, therefor, action for action, HaShem repays such a thing with the covenant of peace to the one that brought peace to HaShem and His people. As we see in the teachings of the sages -
והיה כל ישראל איש אחד וגם מולידיהם יתאהבו ויתאחדו ובמה שביארתי יובן מאמרם בשלהי מסכת עדיות, אין אליהו בא לא לרחק ולא לקרב אלא לעשות שלום ביניהם,
And [then] all of Israel will be one, and also all of the ones that gave birth to them will love each other and be unified. And with what I have elucidated, their statement at the end of Tractate Eduyot is understood, “[Eliyahu] will not come to make distant or to bring close, but to make peace [among them], as it is stated (Mal. 3:23-24), ‘Behold, I will send, etc. [He shall bring back the hearts of the children to their fathers’”
Just like he brings peace between Israel and their Father in Heaven, so too, he shall be worthy of bringing actual peace to the children of Israel.