Arkhandus said:For one thing, the shadows would drastically reduce the Encounter Level, as shown in the DMG, because circumstances would make the battle much easier for the PCs.
Yuerrk. Long post with no breaks in the middle. Sigh... let me see if I can type a suitably short reply before I lose interest.
Here's an example. Party A and Party B is facing the same encounter. Party A is fully rested and has all resources available, because Party A camped out. Party B rushed in with all their once-per-day abilities spent.
The combat resolves, Party A comes out without a scrape. Party B is down to single-digits on HP, and one of their guys is dead, another wound-cursed by a clay golem. Should Party B get more XP, because it was harder for them? No. It could potentially have been a breeze, but for the player's actions.
Party C and Party D faces a different encounter. Party C charges in headlong. The BBEG, supported by all his minions, almost kills them all. Party D spends a whole month scouting the terrain, and picking off minions one by one, until they face the BBeG - who turns out to be a pushover without backup. Should Party D get less XP because it was easy? No. It was easy because of the player's actions. They get exactly the same amount.
This is covered in the DMG somewhere.
Secondly, as the original poster described, his player found out about this stuff but there is no in-game reason/way for the character to have learned this knowledge too.
Certainly. Shadows are 3HD. A Knowledge(religion) check to know about them is DC13. Then you can Shapechange into them. Hmm... here's an assumption I'd have to make: when you Shapechange into something, you know about their Ex and Su abilities, and how to use them. The spell cannot do what it says (grant all Ex and Su abilities) otherwise.
And, in-game, the character has no reason to go looking for this knowledge specifically
The knowledge that some undead can replicate themselves should be rather common. The character would initially see if he could somehow take advantage of that. What Shapechange does would be slightly harder, but in a world where a scroll of Shapechange is available for purchase, not terribly so - especially if he's seen how Polymorph works. Or he could chat with whoever sells them, next time he goes to buy magic gear. With these leads the character can go, 'hey, what if...'
It does take a little RPing, perhaps. But there you go.
On the availability of level 9 spells, see my post above.