A rakshasa disguised amoung other creatures would make a great cameo for level 5-8 before they appear en-masse in gianthold.
Either as an explorer area rare or a rare spawn in a dungeon.
An infernal troglodye SUMMONED by a group of normal trogs would make another good cameo, possibly for tangleroot, the open area trog zone.
Seeing a shadow in searing heights if great fun this is what we need more of. When you're basically NOT prepared to face a once off, instead of being prepared to meet 100's in the shadow crypts.