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.
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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.
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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.