The headword, presumably extracted from the first quotation given, is the masculine nominative singular, future active participle, of the verb katartu/w; cf. generally kappa 757 (katarti/zw).
[1] Sophocles, Oedipus at Colonus 71 (web address 1), with scholion. For molei=n ("to come"), other manuscripts read either ti/ soi ("[to arrange] something for you") or mo/loi, with final w(s, ("that he may come"). For katartu/swn molei=n, Jebb (below ad loc.; web address 2) offers: "to prepare things (to work upon his mind, directly or indirectly), so that he shall come". Compare also Kamerbeek (below ad loc.): "put pressure on"; see also LSJ s.v. 2 (web address 3).
For other uses of katartu/w see, inter alia: Solon 27.11; Sophocles, Antigone 478; Plato, Laws 808D; Apollonius Rhodius, Argonautica 1.392; Plutarch, Moralia 38D.
[2] Homer, Odyssey 11.439 (web address 4). The context is Odysseus' conversation with Agamemnon in the Underworld. In place of do/lon other manuscripts read either xo/lon ("rage") or fo/non ("murder"). Other Homeric uses of a)rtu/w are found at Iliad 18.379 and Odyssey 4.771.