What is the 'Avyay' (indeclinable word) in the sentence: "मैं वहाँ होकर आया हूँ।"?

In this sentence, 'Wahan' (वहाँ) is an adverb of place (Sthanvachak Kriya-visheshon), which falls under the category of 'Avyay' because its form never changes according to gender, number, or tense.