How many letters does the top answer for “of or relating to or characteristic of hades or tartarus” have?

The most common answer is hadean (6 letters).

Crossword clue: of or relating to or characteristic of hades or tartarus

3 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

hadean 6 letters wn-def

the earliest eon in the history of the Earth from the first accretion of planetary material (around 4,600 million years

plutonian 9 letters wn-def

of or relating to or characteristic of Hades or Tartarus

tartarean 9 letters wn-def

of or relating to or characteristic of Hades or Tartarus