| 2001 | Version 74 | Pre-Season | Basic MMORPG gameplay; level up, fight monsters | Korea | Korean | Earliest internal alpha builds, no event logic |
| 2002 | Version 97 | Pre-Season | 👿 Devil Square, Blood Castle, wings level 1–2 | Korea | Korean | Client versions 0.97d and 0.99b appear in forums |
| 2003 | Version 99 | Pre-Season | Dark Lord class, 🏯 Chaos Castle, item +13 | Korea | Korean | SQL dumps contain only basic tables |
| 2004 | Season 0 | 🏰 Castle Siege Dev | CastleSiege.doc authored internally | Korea | Korean | MapServerInfo.dat structure appears in leaked EMUs |
| 2005 | Season 0 | 🏰 Castle Siege Release | 🏰 Castle Siege goes live | Korea | Korean | EventServer handshake logic becomes modular |
| 2006 | Season 1 | Patch Note | 🐺 Crywolf, 🏰 Castle Siege, Ancient items, Fenrir | Korea | Korean | CrywolfEvent.txt leaks with Korean comments |
| 2006 | Season 2 | Patch Note | 💆 Kanturu, 🔷 Jewel of Harmony, SD system, Aida map | Korea | Korean | Socket item logic begins to emerge |
| 2008 | Season 3 Ep1 | Patch Note | Level 3 wings, 🏛 Illusion Temple PvP | Korea | Korean | ⛋ Seed Sphere injection reverse-engineered via DLL hooks |
| 2008 | Season 3 Ep2 | Patch Note | Skill Tree system, Summoner class, UI improvements | Korea | Korean | Skill tree leaks show opcode mismatch with client |
| 2009 | Season 4 | Patch Note | La Cleon map, Raklion, Vulcanus, 🕵 Moss Merchant | Korea | Korean | Socket system formalized with NPCs and spheres |
| 2010 | Season 5 | Patch Note | 👯 Gens system, Fortress Imperial, +15 items | Korea | Korean | Faction-based PvP logic appears in leaked EMUs |
| 2010 | Season 6 Part 1 | Patch Note | Rage Fighter, small wings, MU Helper | Korea | Korean | Skill packets use unique headers (0xC1 0xF3) |
| 2011 | Season 6 Part 3 | Global Release | Global MU Online S6-3 release: Dec 21, 2011 | Global | English | EMU forks align with Webzen release logic |
| 2012 | Ex700 | Patch Note | Elemental system, raised level cap | Korea | Korean | Elemental resistance logic reverse-engineered |
| 2013 | Season 8 | Patch Note | Debenter map, mining system, level cap 730 | Korea | Korean | Mining logic appears in client-side DLLs |
| 2014 | Season 9 | Patch Note | Urk Mountain, Lord Silvester, Pentagrams | Korea | Korean | MUUN upgrade logic reverse-patched |
| 2015–2020 | Season 10–16 | Patch Note | Grow Lancer, Rune Mage, Guardian system | Korea | Korean | DX9+ client breaks compatibility with older EMUs |
| 2021–2023 | Season 17–19 | Patch Note | Kanturu Underground, Atlans Abyss, Companion pets | Global | English | Commercial forks mimic Webzen logic |
| 2024 | Season 20 | Patch Note | Abyssal Temple, Alchemist class, Ethereal weapons | Global | English | IGCN suite includes partial Webzen logic |
| 2025 | Season 21 | Patch Note | Infinity Tower, PvP rebalance, guild fortress building | Global | English | Latest EMU forks reflect Webzen S21 mechanics |
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