You could try using SheepShaver, which is an emulation program and allows the use of OS 9.0.4 or earlier to run Classic apps. Results in doing that with a game like Diablo or Diablo II may be less than satisfactory.
Blizzard has a patch available for Diablo II to install and run it directly in OS X 10.6.8 ot earlier.
If you are in fact wanting to install and run Diablo (not Diablo II), your best bet would be to buy an older Mac, one which can run the needed OS directly. A pre-QuickSilver G4 or G3 should work well for that. Ideally, you would want to get the OS install disks with it.
I can't answer that, since I've never used Parallels, Boot Camp, or any other means of getting Windows onto a Mac. I suspect one of the limiting, perhaps even prohibiting, factors would be what version(s) of Windows can run Diablo. Since I don't use Windows, I can't give suggestions in that regard.
Perhaps someone else who has that knowledge will post in.
Okay. Since you have an i5 processor, that machine probably came with OS X 10.6.3 (Snow Leopard).
You can create a partition small enough for your purposes. To allow sufficient room for saved games (it will need about 100GB per game, one game = one character), I'd suggest a minimum partition size of 60GB; 70GB would be safer. Remember that you can not use all of a volume, you need to leave some free space for the OS to use for file swap, defragging, etc. - 15% free space, minimum 5GB, is recommended.
Step one is to create the partition.
Shut down all running programs except Finder, then start up Disk Utility (Applications > Utilities > Disk Utility).
In the left pane of Disk Utility, select the hard drive item. Not the volume, the one that is probably named Macintosh HD; select its parent; it will have a name that probably includes the manufacturer's designation for the drive.
Once you have that selected, the item "Partition" should become available in the row of function selector buttons in the upper middle of the screen. Click that button, and the partitioning screen should be shown.
In the partition screen should be a physical depiction of the volumes on that drive, a vertical array of boxes. Click the one named the same as your normal volume (probably Macintosh HD). Doing so should select it.
Below that array should be a small "+" button - click it. This will add a new partition, depicted as an empty box in the vertical array.
Use the entry boxes in the middle right to set the size for that new volume (volume = partition) and to set its name if you don't want to use the default name given to it. Be sure the "Format" choice is set to "Mac OS Extended (Journaled)".
Once you're satisfied with the setup, click the Apply button. in the lower right. The new partition will be created, and its icon should appear on the desktop.
Note - this is not a reversible or undoable operation. A newly created partition cn be undone, but it is not easy, and even more difficult to get the space set aside for it back into the main volume. Be sure the set up config is the way you want it to be.
Quit Disk Utility.
Step two is to install Snow Leopard on the new, empty partition. Use your original Install DVD to do that. Boot to it, select the new partition as the destination, install.
Test boot to that partition, and run Software Update to bring it up current. This may take a while.
Once it is up current, copy the Diablo 2 installer that you downloaded from Blizzard to the new partition. When you run it, the OS should state that it needs Rosetta in order to use that program, and should volunteer to fetch it and install it for you. Accept that offer.
Once Rosetta is available, the D2 installer should run; you will then need to have the D2 install disks available, since the installer will ask for them in sequence.
You can switch back and forth from booting to the Lion partition and to the Snow Leopard partition by using Startup Disk (System Preferences > Startup Disk) or on the fly by holding down the Option key during startup.
... i kinda downloaded Diablo II off the internet , so will it still work the same? :O
The install of Diablo 2 for OS X is a sort of hybrid.
The original Mac version of Diablo 2 was designed to run under OS 9; I don't recall if it would run in Classic (OS 9 used as a program rather than as an OS), but I doubt that it would.
Then OS X came along. In order to get D2 to run in OS X, Blizzard needed to do one of two things - rewrite the complete program so it was OS X native; or create a new front end for it that was OS X native but could use the older support files. They chose to do the second.
The OS X patch, which can be downloaded from Blizzard, needs to be installed first. Then, when that is run it acts as an installer, using the original disk set (3 CDs) as source for the files it needs to build. When the installation is complete, you have an OS X native front end (PPC) which is what you start up; it then uses the older support files to run.
Don't know what you downloaded, but if it was not from Blizzard it is almost certainly not authorized, probably even a non-legal copy of someone else's installation. Whether or not that would run well, if at all, is moot.
Manage to find my Diablo II cds when I was clearing up my room the other day however , disk 1 was very badly scratched that's why I decided to try to find a replacement , oh well , ill just get myself a Diablo III mac version . Thank you anyways :) would refer to this instructions when I need to create another partition :)