CREATE TABLE [dbo].[Updates](
[Update_ID] [int] IDENTITY (1,1) NOT NULL,
[User_ID] [int] NOT NULL,
[U_Description] [nvarchar](100) NOT NULL,
[U_Type] [char](1) NOT NULL,
[Item_ID] [int] NOT NULL,
[Date_Time] [datetime] DEFAULT CURRENT_TIMESTAMP NOT NULL,
CONSTRAINT pk_Updates PRIMARY KEY CLUSTERED (Update_ID),
CONSTRAINT fk_Updates FOREIGN KEY (User_ID)
REFERENCES Users (User_ID)
ON DELETE CASCADE
ON UPDATE CASCADE
,
CONSTRAINT fk_Updates2 FOREIGN KEY (Item_ID)
REFERENCES Video (Video_ID)
ON DELETE NO ACTION
ON UPDATE NO ACTION
,
CONSTRAINT fk_Updates3 FOREIGN KEY (Item_ID)
REFERENCES User_Event (Event_ID)
ON DELETE NO ACTION
ON UPDATE NO ACTION

How can i make this work? I use U_Type to know witch table i have to join & Item_ID to find the specific item..

You can't.

You can NEVER declare a foreign key to reference more than one table.

You *CAN* have the fields there and use them as you are doing (that is, Item_ID can be a *NUMBER* that might be in one of several different tables. And you can build queries to do what you want. But because you are violating the database design rules, you won't get SQL Server (or any other database) to help you.

Don't get me wrong: I understand that there are times when you will do things like this because the alternatives are harder to write and even uglier. But you do need to understand that you are doing something that's not supported by standard DB design.