Milku was a god associated with the underworld who was worshiped in the kingdoms of Ugarit and Amurru in the late Bronze Age. It is possible that he originated further south, as Ugaritic texts indicate he was worshiped in cities located in the northern part of the Transjordan region. He was also incorporated into the Hurrian pantheon under the name Milkunni. There is also evidence that he was worshiped in Hittite religion. It is possible that a closely related deity is also known from Mesopotamia.

In the alphabetic script used in Ugarit, which does not always preserve vowels, Milku's name was written the same as the word malku, "king." As a result it is sometimes difficult to tell which of these two cognate words is meant. However, it is agreed that they were vocalized differently. It has been proposed that one of Milku's epithets was a pun referencing this writing convention.

The name of an underworld deity written as mlk in the Ugaritic alphabetic script is typically vocalized as Milku. Manfred Krebernik argues that the vocalization remains uncertain due to the large number of cognates in various Semitic languages, such as Akkadian and Hebrew, which can be used for comparative purposes. However, he considers Milku to be a plausible option based on the syllabic spelling of a Hurrian form of this deity, Milkunni. It is agreed that the name Milku was a cognate of the Ugaritic word malku, "king." Since the words are not identical, Krebernik suggests that the god originated outside the city, though also in an area where another northwest Semitic language was spoken. He notes that the use of a word referring to a king to an underworld deity has a parallel in the etymology of the name of the Mesopotamian god Nergal, the "lord of the great city."

Based on the etymology of Milku's name, Dennis Pardee goes as far as suggesting that in Ugarit he might have functioned as the divine king of the underworld. However, multiple other deities worshiped in this city were also associated with the underworld in addition to him: Resheph, Arsay and Allani. The Hurrian god Nupatik also appears in a single text in the role of a psychopomp. A further deity possibly associated with the underworld also attested in Ugarit was Shuwala. Less certain are the associations between Dagan and Horon and this sphere. Mot, "death," was also associated with the underworld, but was not actively worshiped as a deity, as evidenced by his absence from offering lists.

Milku was referred to as the "eternal king" (mlk ‘lm, malku ‘ālami). Pardee argues that this epithet might refer to the concept of "atemporality of the afterlife." It is additionally possible it was understood as a pun on Milku's name and the word malku. According to Aicha Rahmouni, there is no indication that epithets including the later word necessarily implied a deity was understood as a high ranking member of the pantheon. Milku was also called the "powerful and august god" (‘il gṯr w yqr). The term gṯr most likely functioned as the name of an independent deity in other contexts, but it is unlikely that the god Gaṯaru is meant in this passage. The view that yqr should instead be interpreted as the name of a king, Yaqaru (in the past also erroneously assumed to be the first king of the ruling dynasty of Ugarit known from historical records), is implausible according to Pardee. Rahmouni also considers it impossible that a proper name rather than an epithet is meant. Milku could also be called a hero (rp’u).

It is possible that Milku was sometimes described as a musician, presumably to indicate that at least the afterlife of kings, described in a text presumed to allude to such an activity, was considered joyful.

Unambiguous evidence for the worship of Milku as a distinct deity is present in Ugaritic texts. He is one of the deities invoked in two incantations against snakebite, in which the sun goddess Shapash is asked to summon various deities from their cult centers. The location associated with him in these texts is ‘Aṯtartu. This place name is apparently with the name of a deity, Ashtart. It is assumed that it was located somewhere northwest of the Sea of Galilee, in the Transjordan. The Ugaritic texts invoking Milku from this city appear to reflect a cosmopolitan tradition, as multiple other deities are also invoked from locations outside the kingdom: Tuttul and Mari in Upper Mesopotamia, Larugatu in central Syria, Bibbita in Anatolia, and Caphtor, corresponding to Crete. An echo of this association between Milku and the city of ‘Aṯtartu might be the deity Milkaštart, who is attested in Phoenician and Punic sources.

A second city associated with Milku in Ugaritic texts is Hidra’yu. It has been suggested that ‘Aṯtartu and Hidrayu correspond to the place names Aštarot and Edrei (modern Daraa), associated with biblical king Og of Bashan. In older literature, Hidra’yu was sometimes incorrectly understood as the name of a deity, but this interpretation is now considered grammatically impossible.