I am making a post and beam gazebo. My timbers are 8x10 inches. How could i cut a mortise 8 inches deep? 4" one side and flip? I already have a drop table.
That could work if you can do the 4’'. I think the longest endmill I can buy can cut 50mm or so, so half of what you’d need.
Are they just straight? What about using the CNC to cut the first part accurately and then use a handheld router with a pattern bit?
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Welcome @Joseph_Higgins!
How many mortises, how wide and long? If many, and big mortises, then, already considered renting/buying “chain mortiser” ? Or, if just one-few, maybe CNC a template/jig to get started and start hogging out with router like Jeff suggested.
If angled mortises, then looks like some chain mortiser rigs support that. New are not cheap, could be an interesting project to make, or cautionary tale in the making…
One of the most dangerous tools to use… 
No kidding. I looked up an image and that thing is basically a small chainsaw with guides to plunge down into the wood.
Yeah, they are mostly used in real woodworking shops and they are usually attached to a wall as to not kill someone…
The easiest would be a slotted hole drilling machine, that can’t cripple you that easily. 
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Another option would be to use a drill press with a forstner bit and then chisel out until square.
It would be a lot of work compared to using the LR but it would get the job done in a pinch.
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A 4" bit is long. I’ve done a few projects using 2" dimensional lumber on my Primo, and I struggled some.
Assuming you get past this issue, my next concern would be getting the alignment right when working from both sides. My first thought would be to create a fixture at as know location that has a stub of a tenon (2" maybe). Mounting the first side mortise on tenon stub would give you the precise placement for the second half cut.