The headword noun (and hence the initial glossing one) is in the genitive singular, illustrated by the first and second of the quotations given. It is probably extracted here from the first of them, though many other instances are extant (beginning with Xenophon, Hellenica 5.2.9).
For related words, which are capable of a wide variety of meanings, see epsilon 2644, epsilon 2643, and further references at epsilon 2640.
[1] Polybios 4.4.4 (web address 1).
[2] Polybios fr. 150 Büttner-Wobst. Büttner-Wobst notes (533) that Schweighäuser attributed this fragment to Polybios.
[3] Rather Sophocles, Oedipus at Colonus 536-7 (web address 2), with comments from the scholia. The texts of Sophocles read muri/wn instead of muri/ai, rendering the phrase as "tendings-to of myriad evils." In this quotation the headword is represented in the nominative plural. The word 'convenings' does not appear in the surviving scholia; Bernhardy connects it with Aristophanes, Frogs 1383, where e)pistrofai/ also occurs (web address 3).