What is meant by "make sacrifice of a corpse"?

Solution 1:

Anything can be sacrificed — the modern trope of live sacrifice as being the inherent meaning of “sacrifice” is an unfortunately lurid distortion. (I blame pulp fiction and TV's reliance on bloody sacrifice as a shorthand for “these are bad people”, but I have no cite for that.)

Most online dictionaries limit themselves to the “kill an animal” meaning, but the Merriam online dictionary is more expansive, and even shows the distinction between the meaning that conflates sacrifice with killing and the meaning that doesn't, by including in its fuller definition two senses that are nearly identical except for that conflation:

sacrifice
noun
1 : an act of offering to a deity something precious; especially : the killing of a victim on an altar
2 : something offered in sacrifice

Both senses are primarily the broader meaning of religious sacrifice as an offering of anything. Despite the semantic narrowing to just sacrificial killing that the word is undergoing in modern English use, reflected in sense (1) above, sacrifice can still be correctly and meaningfully applied to something already dead.

When that misunderstanding is dispensed with, it's easy to understand what “make sacrifice of a corpse” means.

The specifics of how a corpse could be made a sacrifice depend inherently on the rites of (real or fictious) religions of the world in question — the most familiar real-world method to the English-speaking world is to burn the sacrifice, in the sense of the biblical “burnt offering.” Other sacrificial rituals may not involve destruction of the offering at all, and may be only symbolic (which in a fantasy setting, can easily be magically/metaphysically real beyond “only” symbolism). In a fantasy setting, such rites could be quite different from modern Western assumptions of what “a religious sacrifice” looks like, limited only by the imagination of the setting's author(s).

To observe this prohibition, then, a follower of the fictitious god Tyr would have to avoid participating in any rites of other in-setting religions that involve sacrificing a corpse.

Solution 2:

In other RPG games (like ADOM, for example) one can pick up corpses and drop them on an altar of a deity as a sacrifice. The sacrifice does not have to be a live one. It can be an artifact, gold, or a corpse.

It seems in the RPG you're describing never sacrificing a corpse is part of showing respect for fallen enemies. Their corpses must not be sacrificed.

It's just an educated guess on my part, as I've never played the game. Hope it helps.