וַיֹּאמֶר֩ אֵלִיָּ֨הוּ הַתִּשְׁבִּ֜י מִתֹּשָׁבֵ֣י גִלְעָד֮ אֶל־אַחְאָב֒ חַי־יְהוָ֞ה אֱלֹהֵ֤י יִשְׂרָאֵל֙ אֲשֶׁ֣ר עָמַ֣דְתִּי לְפָנָ֔יו אִם־יִהְיֶ֛ה הַשָּׁנִ֥ים הָאֵ֖לֶּה טַ֣ל וּמָטָ֑ר כִּ֖י אִם־לְפִ֥י דְבָרִֽי׃ (ס)
And Elijah the Tishbite, who was of the settlers of Gilead, said unto Ahab: ‘As the LORD, the God of Israel, liveth, before whom I stand, there shall not be dew nor rain these years, but according to my word.’
Depths of Yonah
One possible approach lies in the fact that, as Chazal (Sanhedrin 89a) teach, Ein Shenei Neviim MitNabeim BeSignon Echad, no two prophets prophesy in the same style. Thus, sometimes it is appropriate for a prophet of a certain personality to deliver a specific message. For example, Eliyahu HaNavi, who was a fiery zealot (as evidenced in Melachim I 17:1 and 19:10), was the perfect Navi to inform Achav of the impending destruction of his entire family (Melachim I 21:21-24).
Chizkuni
והעמדת אותם, “to present them” to the priests by performing services for them. Use of the root עמד in the in a similar mode is also found Kings I 17,1, where the prophet Elijah saying: ועמדתי לפניו. “I will present myself before him [G-d] as being at his service.”
Rashi
Wouldn’t the name of Heaven have been desecrated: By way of a question. As Eliyahu had sworn (I Kings 17:1), "As the Lord lives [...], there will be no dew or rain except by my word"; and you swore that you would not move until rain falls.