Not being one of the 12 Olympians, Nyx is a relatively unknown goddess with little lore or legends about her. Most of what we have are descriptions of her from other deities, though what we do know is fascinating. Even for a goddess, she was ancient, being among the first children of Chaos and the mother of many who unfortunately ended up more well known than her. That is perhaps why she was such a powerful goddess. Her age and her legacy made her fearsome. While we don't know much, what we do know is that she was strong. Strong enough to terrify Zeus, who was himself obviously exceedingly powerful and feared. In Homer's Illiad 8.15-20 it explicitly states about Zeus' strength:
| ἐξάπτεσθε θεοὶ πᾶσαί τε θέαιναι: ἀλλ᾽ οὐκ ἂν ἐρύσαιτ᾽ ἐξ οὐρανόθεν πεδίον δὲ Ζῆν᾽ ὕπατον μήστωρ᾽, οὐδ᾽ εἰ μάλα πολλὰ κάμοιτε. ἀλλ᾽ ὅτε δὴ καὶ ἐγὼ πρόφρων ἐθέλοιμι ἐρύσσαι, αὐτῇ κεν γαίῃ ἐρύσαιμ᾽ αὐτῇ τε θαλάσσῃ: σειρὴν μέν κεν ἔπειτα περὶ ῥίον Οὐλύμποιο δησαίμην, τὰ δέ κ᾽ αὖτε μετήορα πάντα γένοιτο. τόσσον ἐγὼ περί τ᾽ εἰμὶ θεῶν περί τ᾽ εἴμ᾽ ἀνθρώπων. | If all gods and goddesses took up a golden cord from the heavens, you would not be able to drag Zeus, highest counsellor, from the heavens, though you would try. But if I wished, I could drag you all and the sea up with the earth so that all was hanging from the cord. So great I am above gods and humans. |
My homeric Greek is somewhat rusty, but I think the meaning comes across. Zeus is talking about how much more powerful he is than the others. He is the one saying this, but no one contradicted him on this point. And Nyx terrified him. Later, when Hypnos is talking about fleeing to Nyx to escape Zeus' wrath, we get this in Illiad 14.260:
ὃ δ᾽ ἐπεγρόμενος χαλέπαινε ῥιπτάζων κατὰ δῶμα θεούς, ἐμὲ δ᾽ ἔξοχα πάντων ζήτει: καί κέ μ᾽ ἄϊστον ἀπ᾽ αἰθέρος ἔμβαλε πόντῳ, εἰ μὴ Νὺξ δμήτειρα θεῶν ἐσάωσε καὶ ἀνδρῶν: 260τὴν ἱκόμην φεύγων, ὃ δ᾽ ἐπαύσατο χωόμενός περ. ἅζετο γὰρ μὴ Νυκτὶ θοῇ ἀποθύμια ἕρδοι. | Waking up, he was violent, throwing the house of the gods. He sought me most of all. He would have thrown me to the sea from the heavens unseen, if Nyx, tamer of gods and humans, did not rescue me. I went to her, fleeing his anger. For he stood in awe of swift Nyx and would not anger her. |
What's interesting here is that she is referred to as "δμήτειρα θεῶν [...] καὶ ἀνδρῶν", the "tamer of gods and humans". That's quite an appellation. As well, Zeus himself "stands in awe" of her.
Both of those passages are from the Illiad, though she gets a passing mention in Hesiod's Theogony when the poet describes the gods' family tree at the beginning and when her home is described in line 744, where she lives with some of her children. The LSJ notes that the specific word1(ἅζετο) describing her relationship to Zeus translates to "stand in awe of, esp. gods and one's parents". Zeus is the one who stands in awe. While reading too much into a word, especially without looking at how it is used in multiple contexts, isn't wise, it's interesting to see something like that applied to the literal king of the gods relative to anything else. My interpretation is that it's less of a matter of respect, like one might have for parents, and more a matter of fear, like Greeks had for the gods. Zeus descended from Chaos by way of Gaia, who would have been a sister to Nyx, making Nyx his great aunt. Like all the gods, they were related, but Zeus didn't fear anyone else.
--------
1. http://www.perseus.tufts.edu/hopper/morph?l=a%28%2Fzeto&la=greek&can=a%28%2Fzeto0&prior=per&d=Perseus:text:1999.01.0133:book=14:card=242&i=1