Basil L. Gildersleeve, Syntax of Classical Greek

The neuter singular adjective is often used as the substantive predicate of a masculine or feminine subject, whether singular or plural.

“διαβολὴ γάρ ἐστι δεινότατον”, HDT.7.10, “η”; For calumny is a dreadful evil. “Μυκῆναι μικρὸν ἦν”, THUC.1.10.1(124).

DEM.1.5: “καὶ ὅλως ἄπιστον οἶμαι ταῖς πολιτείαις ἡ τυραννίς”. 19.336: “μὴ λέγ᾽ ὡς καλὸν εἰρήνη, μηδ᾽ ὡς συμφέρον”.

PLATO, Gorg. 506E: “τάξει ἆρα τεταγμένον καὶ κεκοσμημένον ἐστὶν ἡ ἀρετὴ ἑκάστου”; Legg. 663 E: “καλὸν μὲν ἡ ἀλήθεια, ὦ ξένε, καὶ μόνιμον”. Ibid. 732 E: “ἔστι δὴ φύσει ἀνθρώπειον μάλιστα ἡδοναὶ καὶ λῦπαι καὶ ἐπιθυμίαι”. Rpb. 368 E: “οὐκοῦν μεῖζον πόλις ἑνὸς ἀνδρός”;

XEN. Mem. 2.3.1: “χρησιμώτερον νομίζουσι χρήματα ἢ ἀδελφούς” . Oec. 12.15: “κερδαλέον ἐστὶν ἡ ἐπιμέλεια” .

THUC.1.10.1(see above). 1.138.5: “ἐδόκει γὰρ” (sc. “ἡ Δάμψακος”) “πολυοινότατον τῶν τότε εἶναι”. 3.37.3. 4.26.5 (see 124). 4.62.2.8.9.3 (see 124).

HDT.3.108(see 124). 7.10, “η” (see above).

AR. Pl. 203: “δειλότατόν ἐσθ᾽ ὁ πλοῦτος” .

EUR. El. 1035: “μῶρον μὲν οὖν γυναῖκες”. H. F. 1292: “αἱ μεταβολαὶ λυπηρόν”. Or. 232: “δυσάρεστον οἱ νοσοῦντες ἀπορίας ὕπο” . Ibid. 772: “δεινὸν οἱ πολλοί, πανούργους ὅταν ἔχωσι προστάτας” . Suppl. 508: “σφαλερὸν ἡγεμὼν θρασύς”.

SOPH. O. C. 592: “ὦ μῶρε, θυμὸς δ᾽ ἐν κακοῖς οὐ ξύμφορον” .

PIND. fr. 110 Bgk.4: “γλυκὺ δ᾽ ἀπείροισι πόλεμος”, A sweet thing is war to those that have not tried it.

HOM. Il. 2.204: “οὐκ ἀγαθὸν πολυκοιρανίη”.

So a Neuter Relative or other pronoun may have for its antecedent a Masculine or Feminine substantive. See Relative Sentences.