Take one rogue.
Take the feat "ritual casting".
Reflavour some of your exploits if you feel like it. If you're really worried, persuade the DM to relabel some of them spells (good candidates are the utility powers that let you hide in plain view and the like) - it doesn't really matter much though unless you're going to try cheesy stuff like putting rogue powers into wands.
Alternately pick up wizard multiclass.
Buy poison.
Et voila. You're a super sneaky and magically adept poison using rogue, which was all the assassin really ever was.