Relating to or characterized by complacence; marked by smug self-satisfaction or unawareness of problems.
From complacence + -ial (adjective suffix). The -ial suffix, from Latin -ialis, is used to create adjectives meaning 'relating to' or 'having qualities of' the root noun.
The adjective 'complacential' is almost never used in contemporary English, which is telling—we prefer 'complacent' because it's shorter and simpler. This pattern shows how English gradually trims away redundant synonyms, especially when one version is clearly easier to use.
Complete word intelligence in one call. Free tier — 50 lookups/day.