You can use the console to spawn it in. I highly recommend making a separate save before doing this (as I may have forgotten a step). Anyway, the code for the relic is:

x2_p_portal

So, in order to spawn the item, hit Enter to get the cursor into your chat window. Then type:

##DebugMode 1

And hit Enter

Make sure your cursor is in the chat window again and type:

##dm_spawnitem x2_p_portal

and hit Enter again. Your cursor should now turn into targetting mode. Click on your character. This should spawn the item into your inventory (if that doesn't work, just click on a spot on the ground to have it spawn there).

Lastly, get your cursor back into the chat window and type:

##DebugMode 0

And hit Enter. (That's a zero at the end of that command) This will switch off Debug Mode, which is important, as saving the game with it on can corrupt the save.

It's been a while since I played HotU but if I reember rightly the relic is a plot item and thus undroppble. Hoever that said the chapter 2 end boss takes it off you just before he kills you and send you to the hells. Chapter 3 is played without the relic. You'll still be able to get to the reaper, you'll have to find a portal to the reaper's relam though :)