'Sumptuously' is the '-ly' form of 'sumptuous': it tells you that something was done, made, or provided in a rich, expensive, beautiful way. It almost always sits next to a past participle describing how a thing was prepared or decorated — 'sumptuously furnished', 'sumptuously decorated', 'sumptuously illustrated', 'sumptuously produced'. Those pairings are so common that the word is nearly a fixed part of them, and they belong to a particular kind of writing: hotel brochures, auction catalogues, book and film reviews, travel articles. Like 'sumptuous', it carries admiration, not criticism, and it points at things you can see, touch, or taste — cloth, food, rooms, pictures — rather than at abstract things. You can say a house was sumptuously furnished; you would not say someone spoke sumptuously. It is a long, formal word, and in speech an English speaker would more often say 'beautifully' or 'really luxuriously'. Using it well in writing is a small mark of style; using it twice in one paragraph is a mark of trying too hard.