Complex sentence: However, Leo is absent throughout most of season 5 with no explanation; until the episode "The Battle of Evermore" when Hyde gets worried about Leo because he hasn't seen him in a long time.
Split into simpler sentences: However, Leo is absent throughout season 5 with no explanation. During his long absence, he is not mentioned until the episode "The Battle of Evermore" when Hyde gets worried about Leo because he hasn't seen him in a long time.