Leviticus 16:2

So he will not die, the way his sons did.

For if he comes (into the Holy of Holies) he will die

And the Lord said unto Mosheh: Speak with Aharon thy brother, that he enter not at any time into the holy place within the veil before the mercy-seat; for the cloud of the glory of My Shekinah is revealed over the place of the mercy-seat.

This passage provides conclusive evidence that the children of Aaron offered the incense within the Partition. into the Holiness relative to the rest of the Tent of Assembly

in the cloud They may enter only after making a cloud of incense, so that they do not see the Glory and thereby die — that is to say: I shall not be seen by you, except in the cloud . Others explain the phrase to mean, “because I dwell in the cloud which is upon the ark-cover ”, in the sense of “God said that he would dwell in the thick darkness” [I Kings 8:12].

SPEAK UNTO AARON THY BROTHER. The meaning of the epithet thy brother is that “you are to warn him because he is your brother, for even though you are not under this prohibition against coming [into the holy place at all times, thus you might think that Aaron, too, is not subject to this restriction, since he is your brother, nonetheless] Aaron, your brother, is under the prohibition against coming [into the holy place at all times].”
Now Rabbi Abraham ibn Ezra commented that this Scriptural section indicates that Aaron’s sons [died because they] brought the fire of incense into the innermost part of the Sanctuary. (M’kor Chayim). But in my opinion this is not correct. For the verses which mention their sin always say, when they offered ‘strange’ fire before the Eternal., 26:61. And if [you accept as] proof that they entered into the innermost part of the Sanctuary, the prohibition with which G-d warned Moses concerning their father [i.e., Aaron], so that he die not [as Ibn Ezra said], then [you should] certainly accept as proof [that they died because] they entered the Sanctuary whilst intoxicated by wine, the prohibition that was said to Aaron himself immediately after their death, [Drink no wine nor strong drink etc.]! Moreover, how could it have occurred to them to enter on that day the [innermost] part [of the Sanctuary] which [even] their father did not enter, for Aaron burnt the incense on the “inner” altar [which stood in the Sanctuary proper], and why should they bring in their incense to a place further inside [the Sanctuary] than their father! Now I have already hinted at the nature of their sin, and the language of the verses point thereto. But the expression ‘b’korvatham’ (when they approached) [does not mean, as Ibn Ezra interpreted it, “when they approached by entering the innermost part of the holy place”, but] according to its plain meaning is like the expression ‘uv’korvatham’ (and when they come near) to the altar to minister. (ibid., 30:20). Thus the verse here is stating that Aaron’s sons died when they ministered before G-d. If so, [the sense of the warning here] is that He warned Aaron that he should only minister in the place which He commands [that it be done], and at the time He specifies for it.
It is possible that the sense of the verse is similar to that which our Rabbis have said, 6. See also Rashi, Numbers 17:13. that the people were speaking perversely of the incense, saying, “Through it Nadab and Abihu [Aaron’s sons] died etc.” Therefore Scripture stated that after the death of the two sons of Aaron, when they drew near before the Eternal with the incense, He said to Aaron that he should come even nearer before the Eternal than they did, and with incense, for if he comes into the holy place [i.e., the Holy of Holies] without the incense, he will die, for with it he shall enter there first, just as He said, and he shall bring it within the Veil … and the cloud of the incense will cover the ark-cover that is upon the testimony, that he die not. This then is the meaning of the phrase [here in Verse 2 before us], for I will appear in the cloud upon the ark-cover, meaning that he [Aaron now, and the High Priest in succeeding generations] is only to enter there with the incense whose cloud rises up there [upon the ark-cover], even as He said, and the cloud of the incense will cover the ark-cover.

כי בענן אראה על הכפורת. According to the plain meaning of the text, we are told that G’d would become manifest by means of the cloud which hovers over the spot where the lid of the Holy Ark is situated within the Tabernacle. Compare Exodus 25,22 where G’d told Moses: “I will speak with you from above the כפורת from the area between the two cherubs.” If the priest would see this area with his own eyes he would die. Therefore, when he would enter the Tabernacle (Holy of Holies) on the Day of Atonement to burn the incense and sprinkle the blood of the bull and ram of the sin offering on the dividing curtain, the cloud would first wrap the entire Tabernacle into darkness so that he would not be able to see what he was not allowed to see.

FOR I APPEAR IN THE CLOUD in this generation to speak to Moshe, but in other generations to call to those prepared for prophecy, as was the case with Shmu'el, as it says (I Sam. 3:3-4), "Shmu'el was sleeping in the temple of Adonai, where the Ark of God was, and Adonai called out to Shmu'el.