wolfen said:I have been scorned for my reluctance to accept that there is NO FACING in D&D. Right? No front, no back. Everyone on a battlefield is assumed to be scanning 360 degrees, etc. This is why a Monk deflecting arrows doesn't have to declare which direction he's facing. This is why (ridiculous as it is) you don't get any bonus for attacking someone's back and why a rogue doesn't get a sneak attack by sneaking behind someone...there's no behind.
No more ridiculous than telling the fighter "sorry, even though you know the unhidden rogue is there, he gets to walk 30 feet around behind you and then stab you in the back with huge bonuses to hit and damage."
Because, you know, when a trained warrior is attacking 2 men, rogue and fighter, he'll always ignore one to attack the other one, turning his back...

So that being the case for combatants, shouldn't it be true for spellcasters? There is another thread asking about "Cloudkill" and how the wizard simply casts in the direction he's facing...but that runs into the problem. He's not definitively facing anywhere, and there's not a rule-defined mechanism for him to assert one.
SO, let's say a Spellcaster is casting Fireball or Lightning Bolt. Let's say he didn't declare a target when he first announced the spell to the DM and he is suddenly struck blind before releasing the spell -- he can't target...
As you say, this is some house rule in another thread, not something the designers put in the PH or DMG...
Of course every once and a while situations will come up where rules don't cover something a character wants to do, and a DM and/or player will have to decide something based on the story.
DM "You're blind"
Player "I want to fireball the area where I last saw that group of undead"
DM "I don't see a rule for targetting while blind"
Player "Well, I was planning to do that anyway, so I probably would already have a decent idea where they are"
DM "Okay, I'll roll an 8-sided to see how far off your intended target you are. 1-2 on target, 3-4 off by 1, 5-6 off by 2, 7-8 off by 3"
Player "Sounds fair"
or whatever.
DM2