Sant'Erasmo is a ruined mediaeval oratory on the Via delle Canapiglie in the suburb of Torre Maura (the zone is actually Torre Spaccata).
The dedication is to St Erasmus.
Info.roma has a page here on this ruin, which has a single high wall in random rubble. It is visible through the trees, at the top of a bank on the north side of the street near the north junction with the Via delle Pispole. This is the south end of the Parco delle Canapiglie.
The writer has not been able to find out anything about it.
Monastery of Sant'Erasmo al Celio Edit
There was a monastery dedicated to the saint in the city in early mediaeval times, Sant'Erasmo al Celio, which was on site near Santo Stefano Rotondo al Celio. This is the only other dedication that the saint has had in the city. Was there a link?
This monastery seems to have been founded by Byzantine-rite expatriate monks from the East, although the circumstances are wholly unrecorded.
Rome was first colonised by such monks during the invasion of the Eastern Roman Empire by the Sassanid Persian Empire from 602 to 628, when the Persians briefly conquered Syria, the Holy Land and Egypt. They were followed by the Muslims, who conquered Jerusalem in 637. So many monastic refugees arrived in Rome that they dominated Roman monasticism for the next one and a half centuries, and it was they who seem to have overseen the arduous but wholly undocumented task of converting the hills of Rome within the city walls from ruined residential areas to vineyards.
It needs to be borne in mind that the presence and dominance of these monks was maliciously airbrushed out of the history of the Roman church later on, especially after the Great Schism of 1054. The myth that Roman monasticism was Benedictine (that is, always Latin rite) since the later 6th century was then sedulously propagated, and unfortunately still finds influence.
The first mention of the Greek-speaking (almost certainly Byzantine-rite) monastery of Sancti Herasmi situm in Celiomonte is in the Liber Pontificalis entry for Pope Adeodatus II (672-6), which states that he resided at the monastery and endowed it. Later tradition held that he was a monk there (see below).
There has been confusion among modern scholars with another monastery on the hill called Sant'Abbaciro al Celio, which was dedicated to the senior of the two martyrs SS Cyrus and John. See Santa Passera.
An epigraph in Greek dating from the 7th century mentions the monastery as being in possession of a property on the Via Appia just outside the city gate (this place was named as Marmoratoula in Greek).
The Itinerarium Einsiedeln, a pilgrim guide possibly originally of the 8th century, placed the monastery Sancti Erasmi between the church of Santo Stefano Rotondo al Celio and the Aqua Claudia aqueduct, which focused suspicion on the site of the present Ospizio dell'Addolorata. However, alternative suggestions are next to San Tommaso in Formis or south of Santo Stefano -and it's not possible to decide between these without a future archaeological discovery.
In 799 the monastery gave Pope Leo III shelter from his enemies, and he rewarded it with gifts. This action was repeated by Pope Gregory IV (827-44), but this is the last record of the Greek monastery and the circumstances in which the Byzantine rite monks left are again wholly unknown.
The first firm evidence of Sant'Erasmo as a Benedictine Latin rite monastery dates from 938, when Pope Leo VII confirmed its possession by the abbey of Subiaco, and the previous year when it was listed as a possession in the Registrum Sublacense which was a list of charters compiled for that monastery. Unfortunately the abbey then created a tendentious and basically fictional foundation legend, backing it up by forging documents, and this resulted in the so-called Chronicon Sublacense published in the 17th century. This disgraceful piece of falsehood is unfortunately still influential as a "historical source".
According to it, "the monks of Subiaco (begging the question as to whether the monastery existed then) went into exile at Sant'Erasmo after their own monastery was destroyed in a barbarian raid in 601, and stayed there until 701 when they went back home. Thus Pope Adeodatus II was a Benedictine monk, and so was the monastery from then on." Thus the Greek monks were given the Stalinist historiographical treatment.
Sant'Erasmo functioned as a dependent priory of Subiaco from 938 until some time in the late 12th century. Then, Benedictine monastic life in the city collapsed completely and the Order went from having twenty abbeys at Rome down to one (San Paolo fuori le Mura). The cause was that the monks had become completely degenerate in their way of life and were posing as noblemen, to the extent of taking mistresses and having children. At Sant'Erasmo, they were replaced by Benedictine nuns at an unknown date -the Catalogue of Turin listed the convent as having sixteen nuns in about 1320.
The nunnery in turn became degenerate, and failed in the 15th century. It was apparently annexed to the hospital of San Tommaso in Formis, but the last listing was in 1555 and it is thought that the church was demolished soon after.
The site of the church remains unknown.
Annas Rom Guide (in Danish)