He shall come to know
Dionysus, son of Zeus, consummate god,
most terrible, and yet most gentle, to mankind.

[γνώσεται δὲ τὸν Διὸς
Διόνυσον, ὃς πέφυκεν ἐν τέλει θεός,
δεινότατος, ἀνθρώποισι δ᾽ ἠπιώτατος.]

Euripides (485?-406? BC) Greek tragic dramatist

Bacchæ [Βάκχαι], l. 859ff [Dionysus/Διόνυσος] (405 BC) [tr. Arrowsmith (1960)]
    (Source)

Speaking of King Pentheus. (Source (Greek)). Alternate translations:

Thus he shall know dread Bacchus, son of Jove,
A god most terrible when he asserts
His slighted power: but gracious to mankind.
[tr. Wodhull (1809)]

He will recognize the son of Zeus, Dionysus, who is in fact a god, the most terrible and yet most mild to men.
[tr. Buckley (1850)]

Know he must
Dionysus, son of Jove, among the gods
Mightiest, yet mildest to the sons of men.
[tr. Milman (1865)]

There belike to tell
That Dionysus, son to Zeus, is god,
Most terrible, most gracious unto men.
[tr. Rogers (1872), l. 820ff]

So shall he recognize Dionysus, the son of Zeus, who proves himself at last a god most terrible, for all his gentleness to man.
[tr. Coleridge (1891)]

And he shall know Zeus' son
Dionysus, who hath risen at last a God
Most terrible, yet kindest unto men.
[tr. Way (1898)]

So shall he learn and mark
God's true Son, Dionyse, in fulness God,
Most fearful, yet to man most soft of mood.
[tr. Murray (1902)]

And he shall recognize the son of Zeus,
Dionysus, as a god in perfect essence:
a terrible one, but to men most gentle.
[tr. Kirk (1970)]

And he shall know the son of Zeus, Dionysus; who, those most gentle to mankind, can prove a god of terror irresistible.
[tr. Vellacott (1973)]

Consummate god, most terrible, most gentle
To mankind.
[tr. Soyinka (1973), Bacchante speaking]

He shall know Zeus’ son
Dionysos, that he is in his fullness a god
most dreadful, and to men most mild.
[tr. Neuburg (1988)]

So shall Pentheus come to know Dionysus, son of Zeus,
a God sprung from nature, like nature most cruel,
and, yet, most gentle to mankind.
[tr. Cacoyannis (1982)]

And he'll know
Zeus-born Dionysos is a true divinity,
Most terrifying to men, and most kind.
[tr. Blessington (1993)]

He will come to know Dionysus, the son of Zeus,
that he is, in the ritual of initiation, a god most terrifying,
but for mankind a god most gentle.
[tr. Esposito (1998)]

Then he will know the son of Zeus,
Dionysus, and realize that he was born a god, bringing
terrors for initiation, and to the people, gentle grace.
[tr. Woodruff (1999)]

And he will know that Dionysos, son
Of Zeus, was born a god in full, and is
Most terrible to mortals and most gentle.
[tr. Gibbons/Segal (2000)]

He will learn that Dionysus is in the full sense a god, a god most dreadful to morals -- but also most gentle!
[tr. Kovacs (2002)]

He'll learn the nature of this son of Zeus:
The sweetest and most fearsome of the gods.
[tr. Teevan (2002)]

Only then will he learn that the son of Zeus, Dionysos, is a god of peace for the good folk but he is also a fearsome god who those who don’t respect him.
[tr. Theodoridis (2005)]

He will recognize Zeus' son Dionysus, born in ritual,
The most terrible god -- and kindest to humans.
[tr. Valerie (2005)]

He'll come to acknowledge
Dionysus, son of Zeus, born in full divinity,
most fearful, yet most kind to human beings.
[tr. Johnston (2008)]

And he shall finally know Dionysus, son of Zeus,
a god both terrible and gentle to the world of man.
[tr. Robertson (2014)]

He will know Dionysus. He will know the son of Zeus to be true-god-born, to be the greatest horror to mortal kind.
And the greatest helper.
[tr. Pauly (2019)]

He shall learn that Dionysus is the son of Zeuis, a god with the power of a god, a god most fearful and most gentle.
[tr. Behr/Foster (2019)]

And he will come to know the son of Zeus,
Dionysus, the one who is by his own nature a god in the end [telos],
the one who is most terrifying [deinos], but, for humans, also most gentle [ēpios ].
[tr. Buckley/Sens/Nagy (2020)]