Some online sources on Greek myth you can find will bring up a fragment of anonymous authorship which has been interpreted as saying that Hades is the master of the Oneiroi, basically personified dream spirits. On sites such as Theoi, you can find the fragment displayed as such, allegedly pulled from from Loeb's Greek Lyric, Volume V: The New School of Poetry and Anonymous Songs and Hymns, translated by David A. Campbell:
"Plouton (Pluto) [Haides], master of the black-winged Oneiroi (Dreams)."
However, here is the fragment as it is actually presented in the book:
Greek:
γίγνονται δὲ καὶ ἀπὸ λέξεως χάριτες ἢ ἐκ μεταφορᾶς, ὡς ἐπὶ τοῦ τέττιγος (Alc. 347b), ἢ ἐκ συνθέτου του ὀνόματος καὶ διθυραμβικοῦ·
δέσποτα Πλούτων μελανοπτερύγων· τουτὶ δεινὸν †προπτερύγων αὐτὸ ποίησον†, ἃ μάλιστα δὴ κωμῳδικὰ παίγνιά ἐστι καὶ σατυρικά (Gale: σατύρια cod.).
English:
Literary grace may be due to the choice of words or to metaphor, as in the lines on the cicada (Ale. 347b), or to a compound word of dithyrambic type:
Pluto, master of the black-winged [1] . . . !
. . . these are for the most part the jokes of comedy or satyr-plays.
Bergk: πλοῦτον cod. post μελ. <ὀνείρων> Berkg, <ψυχῶν>
[1]: Dreams? Ghosts? Text corrupt.
As you can see, the text is corrupt, and different translators have chosen to reconstruct it in different ways. The fragment is pulled from On Style, a treatise on literature attributed to the writer Demetrius. The dating of the text is unclear; some argue it's from the 1st century AD, others argue it has earlier origins around the 2nd century BC. Loeb's translation of On Style, by Doreen Innes, translates the passage which includes the fragment differently, and the notes show the corruption more specifically:
Greek:
πολλὰς δ᾿ ἄν τις καὶ ἄλλας ἐκφέροι χάριτας. Γίγνονται δὲ καὶ ἀπὸ λέξεως χάριτες, ἢ ἐκ μεταφορᾶς, ὡς ἐπὶ τοῦ τέττιγος, “πτερύγων δ᾿ ὑποκακχέει λιγυρὰν ἀοιδάν, ὅτι ποτ᾿ ἂν φλόγιον †καθέταν ἐπιπτάμενον καταυλεῖ,”
ἢ [167] ἐκ συνθέτου τοῦ [168] ὀνόματος καὶ διθυραμβικοῦ, “δέσποτα Πλούτων [169] μελανοπτερύγων—τουτὶ δεινόν, πυρροπτερύγων [170] αὐτὸ ποίησον.” ἃ μάλιστα δὴ κωμῳδικὰ παίγνιά ἐστι καὶ σατυρικά. [171]
[167]: locus corruptus, at recte καταυλεῖ· ἢ Finckh: καταυδείη P.
[168]: τοῦ del. Finckh
[169]: Πλούτων M^2 Bergk: πλοῦτον P.
[170]: πυρροπτερύγων Wilamowitz (qui et personas distinxit): πρὸ πτερύγων P.
[171]: σατυρικά Gale: σατύρια P.
English:
Charm also comes from the use of a single word, for example from metaphor, as in the passage about the cicada, “from under his wings he pours out a stream of piercing song, as in the blazing heat of summer he flies and flutes.”
or from dithyrambic compounds, “‘Pluto, lord of the sable-winged’—‘that is terrible, make it red-winged.’” [175] Such freaks of language are best suited for comedy and satyr drama.
[175]: Author unknown, presumably comedy (Supp. Com. Adesp. 1) rather than lyric (PMG 963). The text is corrupt, but parodies tragic compounds in “-winged,” and is probably a dialogue.
The choice to interpret the corrupted area as "dreams" possibly stems from the descriptor "μελανοπτερύγων" or "black-winged" being applied to dreams in other sources, such as here in Euripides' play Hecuba, written around 424 BC. Both the Greek and English text are taken from Loeb's translation by David Kovacs:
Greek:
δείμασι φάσμασιν; ὦ πότνια Χθών,μελανοπτερύγων μᾶτερ ὀνείρων,ἀποπέμπομαι ἔννυχον ὄψιν[ἣν περὶ παιδὸς ἐμοῦ τοῦ σῳζομένου κατὰ Θρῄκηνἀμφὶ Πολυξείνης τε φίλης θυγατρὸς δι᾿ ὀνείρων†εἶδον γὰρ φοβερὰν ὄψιν ἔμαθον ἐδάην†].ὦ χθόνιοι θεοί, σώσατε παῖδ᾿ ἐμόν,]ὃς μόνος οἴκων ἄγκυρ᾿ ἔτ᾿ ἐμῶντὰν χιονώδη Θρῄκαν κατέχειξείνου πατρίου φυλακαῖσιν.ἔσται τι νέον·ἥξει τι μέλος γοερὸν γοεραῖς.οὔποτ᾿ ἐμὰ φρὴν ὧδ᾿ ἀλίαστονφρίσσει ταρβεῖ.ποῦ ποτε θείαν Ἑλένου ψυχὰνκαὶ Κασσάνδραν ἐσίδω, Τρῳάδες,ὥς μοι κρίνωσιν ὀνείρους;[εἶδον γὰρ βαλιὰν ἔλαφον λύκου αἵμονι χαλᾷσφαζομέναν, ἀπ᾿ ἐμῶν γονάτων σπασθεῖσαν ἀνοίκτως.]καὶ τόδε δεῖμά μοι·ἦλθ᾿ ὑπὲρ ἄκρας τύμβου κορυφᾶςφάντασμ᾿ Ἀχιλέως· ᾔτει δὲ γέραςτῶν πολυμόχθων τινὰ Τρωιάδων.ἀπ᾿ ἐμᾶς ἀπ᾿ ἐμᾶς οὖν τόδε παιδὸςπέμψατε, δαίμονες, ἱκετεύω.
English:
O lady Earth, mother of black-winged dreams, I thrust from myself the vision of this night [which I saw in dreams concerning my son kept safe in Thrace and about Polyxena my dear daughter, for I saw, beheld, learned of a fearful vision]! O gods of the nether world, spare the life of my son! He is the last remaining anchor of my house and dwells in snowy Thrace in the keeping of his father’s guest-friend. Some new sorrow shall come to pass. A tearful strain will come to those already in tears. Never has my heart been so unabatingly fearful and anxious. Where can I see Helenus, that prophetic soul, or Cassandra so that they may interpret my dreams? [For I saw a dappled doe, its throat being cut by the bloody paw of a wolf, torn pitilessly from my knees.] I have this fear as well: the ghost of Achilles came and stood above the peak of his tomb. And he kept asking for one of the troubleladen daughters of Troy as a prize of honor. From my daughter, from my daughter, I entreat you, O gods, avert this fate!
Additionally, the tie between Hades and dreams can be seen in earlier sources such as the Odyssey, where the "land of dreams" is portrayed as being on the way to the underworld. The text here is pulled from Book 24 of the Odyssey, A. T. Murray's 1919 translation:
Meanwhile Cyllenian Hermes called forth the spirits of the wooers. He held in his hands his wand, a fair wand of gold, wherewith he lulls to sleep the eyes of whom he will, while others again he wakens even out of slumber; with this he roused and led the spirits, and they followed gibbering. And as in the innermost recess of a wondrous cave bats flit about gibbering, when one has fallen from off the rock from the chain in which they cling to one another, so these went with him gibbering, and Hermes, the Helper, led them down the dank ways. Past the streams of Oceanus they went, past the rock Leucas, past the gates of the sun and the land of dreams, and quickly came to the mead of asphodel, where the spirits dwell, phantoms of men who have done with toils.
It's worth noting, however, that some translations of the fragment will seemingly interpret it differently and instead apply the descriptor of "black-winged" to Hades himself, such as the translation by Ian C. Storey found in Loeb's Fragments of Old Comedy, Volume III:
Greek:
{A.} δέσποτα Πλούτων μελανοπτερύγων,
{Β.} τουτὶ δεινὸν. π<υρ>ροπτερύγων αὐτὸ ποίησον
English:
(A) “Pluto, black-winged Lord.”
(B) That’s awful. Make him “with wings of fiery red.”
In all, there's some confusion around this fragment, and it's unclear what exactly it's meant to mean.