Recently in a game the issue arose of whether a Wizard is affected by his/her own casting of the Evard's Black Tentacles spell. We all agreed that both enemies and allies alike that are located within the area of effect are targeted by the tentacles' grapple attempts. However, what if the caster applies the spell to an area he/she is currently standing in, or if he/she moves into the area of effect? We could not agree on a ruling, and no one could find a rule in the books to settle the issue. In the interest of avoiding conflict and saving time, we decided to put the issue on the back-burner and allowed the Wizard to avoid the tentacles, however it would be helpful in future games to have a rules-as-written supported position.
Yes, Evard’s black tentacles affect anyone they can get their... tentacles... on
From the spell itself:
Every creature within the area
This does not leave room for excepting allies, including yourself. Area spells, by default, affect all in the area and do not let give you immunity. See Aiming a Spell – Area:
Regardless of the shape of the area, you select the point where the spell originates, but otherwise you don’t control which creatures or objects the spell affects.
Area: 20-ft.-radius spread
Note the lack of an (S) which would indicate that the caster can shape the area, and leave a square out of it.
Yes, the caster is affected by his own spell. The wording of the spell itself gives you everything you need to know. It specifically says:
Every creature within the area of the spell must make a grapple check
Any creature that enters the area of the spell is immediately attacked by the tentacles
Quotes are directly from the SRD on Black Tentacles, but I assume the spell description in the book is the same.