ReBorn HD is the biggest balance patch-mod for Sacred Underwold (Sacred Gold) version 2.28. Rebalanced almost all big bosses, rebalanced a lot of Combat Arts, global difficulty balance, items rebalance, new mechanics, improved drop and bugfixes, new difficult chalanges in several caves.
See changelists for details, because it’s hard to describe everything briefly. See "by versions" changelist if you want to know what's new in latest version. See "total" changelist if you want to see all actual changes.
Links for actual installer and changelists you can find: