King of all the Isles

Needs Northern Lords, and it's a genuine monster: every listed Atlantic and Mediterranean island - the duchies of Iceland, the Northern Isles, Sjælland, Visby, the Isles, Mann, Canarias, Mallorca, Sardinia, Corsica, Sicily, Venice, Cephalonia, Krete and Cyprus, plus the counties of Bornholm, Aland, Öland, Osel, Anglesey, Wight, Chios, Lesbos, Naxos and Rhodos - as a North Germanic Ásatrúan who never exceeds 80 realm size. As Hæsteinn (867): take part of Sardinia for diplomatic range, then invade Thessalonika immediately with mercs to grab its islands and cripple Byzantium. Do NOT form a kingdom title - it blocks the Elevate the Kingdom of Mann & the Isles decision, whose event troops carry the rest of the campaign. The realm-size condition was famously coded backwards at launch in 2021 (acknowledged and long since sorted), which explains the achievement's terrifying reputation; nowadays it's merely very hard.