Diadumenian Emperor of the Roman Empire from 218 to 218.
Marcus Opellius Antoninus Diadumenianus Augustus was the son of the Roman Emperor Macrinus, and served his father briefly as Caesar (May 217–218) and as Augustus (in 218).

The name stems from him being born with a caul that formed a 'diadem'.

Diadumenian had little time to enjoy his position or to learn anything from its opportunities because the legions of Syria revolted and declared Elagabalus ruler of the Roman Empire. When Macrinus was defeated on 8 June 218, at Antioch, Diadumenian's death followed his father's.
Diadumenian
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An AE unit struck 217-218 AD in Nicopolis ad Istrum
Obverse: bare head right; K M OΠΠEΛ ANTΩNI ΔIAΔOVMENIANOC

Reverse: Homonoia facing, head left, holding patera and cornucopia; VΠ AΓPIΠΠA NIK_OΠOΛITΩN ΠΡOCIC / TΡΩ

Diameter: -
Die Orientation: -
Weight: 12.4 g
Agrippa, consular legate
AMNG I 1817f. (Var.); Varbanov 3641
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An AE unit struck 217-218 AD in Nicopolis ad Istrum
Obverse: Κ Μ ΟΠΠΕΛ ΑΝΤΩΝΙ ∆ΙΑ∆ΟΥΜΕΝΙΑΝΟC, Bare bust of Diadumenian facing right

Reverse: VΠ ΑΓΡΙΠΠΑ ΝΙΚΟΠΟΛΙΤΩΝ ΠΡΟC ICT, City-gate flanked by two towers and with a taller third tower rising from center (or from behind)

Diameter: 28 mm
Die Orientation: 12 H
Weight: 13.72 g

Diadumenian as Caesar under his father Macrinus as Augustus

Marcus Claudius Agrippa as legatus consularis

Varbanov 3639
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An AE unit struck 217-218 AD in Caesarea Maritima
Obverse: draped and cuirassed bust right; M OP DIADVMENIAN[V?] CAE

Reverse: Serapis head right wearing modius; COL·P·FL·AV_F·C·CAESAR

Diameter: -
Die Orientation: -
Weight: 7.7 g
No notes for this coin
Kadman 78; SNG ANS 801; Rosenberger 67.
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An AE AE17 struck 218 in Nicopolis ad Istrum
Obverse: K M OΠΠEΛ ANTΩNINOC, bare-headed, draped bust right, seen from the back

Reverse: NIKOΠOΛITΩN, serpent-entwined staff of Asklepios

Diameter: 18 mm
Die Orientation: -
Weight: 1.9 g
No notes for this coin
AMNG 1885; Varbanov 3598 var (bust type); Megaw NIC1.2e
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An AE unit struck 217-218 AD in Nicopolis ad Istrum
Obverse: draped and cuirassed bust right from behind; K M OΠΠEΛ__ANTΩNINOC

Reverse: Asklepian snake-encircled staff; NIKOΠO_ΛITΩN

Diameter: -
Die Orientation: -
Weight: -
No notes for this coin
Varbanov 3598 var. (bust type)
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An AR Tetradrachm struck 217-218 AD in Antioch
Obverse: Cuirassed bust right; KAIC•M•OΠ•ΕΛ•ANTΩNINOC

Reverse: Eagle standing facing on leg and thigh of animal, head and tail right, with wings spread, holding wreath in beak; ·Δ·_·Є· across upper field; ΔHMAPX•ΕΞ•YΠA•TO

Diameter: -
Die Orientation: -
Weight: -
No notes for this coin
McAlee 744; Prieur 248 corr. (obv. legend)
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An AE unit struck 218 AD in Nicopolis ad Istrum
Obverse: M OΠEΛ ΔIAΔOVMENIANOC KAI / bare-headed, draped and cuirassed bust right

Reverse: VΠ CTA ΛONΓINOV NI[KOΠOΛITΩN] ΠPOC IC / Nemesis-Dikaiosyne standing facing, head left, holding scales and sceptre; wheel at feet

Diameter: 26 mm
Die Orientation: -
Weight: 16.23 g
No notes for this coin
Varbanov 3725 var. (obv. leg.).
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An AE Pentassarion struck 217-218 AD in Marcianopolis
Obverse: AV K OΠEΛ CEVH MAKPEINOC K M OΠEΛ ANTΩNEINOC / Laureate, draped and cuirassed bust of Macrinus right facing bare headed, draped and cuirassed bust of Diadumenian left

Reverse: VΠ ΠONTIAN-O-V MAPKIANOΠOΛEI / TΩ-N / Demeter standing right, holding 2 corn-ears in left hand and a sceptre in right. E in upper right field

Diameter: -
Die Orientation: -
Weight: 11.9 g
No notes for this coin
AMNG I/1, 725 (1 ex. in München), Varbanov (engl.) 1262