The tomb of the Abbasid caliph Harun al-Rashid is the original reason Mashhad exists as a city. The site is inside the Imam Reza Shrine complex itself, immediately next to the imam's tomb, though the focus of veneration has always been Imam Reza. Sunni visitors come specifically to pay respects to Harun.
The mausoleum is open as part of the wider shrine visit. The political and theological weight of the two tombs sitting side by side — the Sunni caliph and the Shia imam — is the more interesting story for any visitor with a sense of history.