1980ish book, runestone, Tolkien elvish

This is a somewhat forgettable (obviously) book I read in paperback, 1980 or 1981.

I believe it was set in the modern-day. Someone discovers a runestone. Somehow, (possibly involving blood sacrifice or ritual?) it opens a gateway to another world (or time?). Some kind of killer (berserker? warrior of some time) may have come through. That's about all I remember of the plot.

The most memorable thing about it is that the language used in this other world was Tolkien's Elvish (probably Sindarin). I remember that I was able to translate it with just the bits I'd picked up from reading LoTR and The Silmarillion many times. Even to my 14-15 year-old self, this didn't seem quite right to me - a bit of a ripoff of JRRT.