A Cleric5/Malconvoker5 needs to cast Desecrate. He is currently Good aligned and wants to maintain that alignment in order to avoid complications from pesky adventurers' overzealous use of Detect Evil.
He can cast up to Summon Monster V, so the easiest way is probably to simply summon something to cast the spell for him but the basic summon lists don't seem to have anything with it as a SLA.
1) If the Cleric is a PC his alignment is his choice and can change suddenly at any time. As long as his God is okay with it, he can suddenly turn evil, cast the spell, then turn good again according to the RAW in the atonement spell description.
2) If that's not allowed he can use the Atonement spell, but since he's deliberately casting an evil spell, he will have to pay the 500 XP penalty cost. This is pretty reasonable as a get-around-a-class-restriction fee.
3) The Cleric can hire a buddy, preferably another Cleric of the same God, who's less undercover about being evil. This costs ~87.5 gp, but you can probably get it for less if you just time-share with a necromancer-Cleric who was gonna be doing it anyways. The latter case assumes you don't need this done on a specific area.
4) Summoning's no good here, since none of the summons available at Summon Monster V or below have access to the spell. You CAN use Lesser Planar Ally to summon a Neutral or Evil Aligned level 3 Cleric of your God who currently has Desecrate prepared, but this'll end up costing you 100gp and 100XP, plus the cost listed in number 2, so it's not a good idea unless there's no similarly-aligned evil Clerics around for hire.
5) You could find a Nightshade and try to sneak up close to it or manipulate it into going where you need it or make a deal with it, but this'll probably just end with you (un)dead. Unless you are already, in which case this is probably legit. You can do this with creatures merely capable of casting desecrate (perhaps at will), but then we're clearly back to number 2.
6) You could try doing it the old fashioned way, where you brutally murder a hundred innocent (preferably high Cha virgins) villagers and scrawl in their poo and blood prayers to your demonic masters and insults to everyone in general upon the target of your desecrate, while burning long candles of their fat embedded in the piles of their skulls in a pentagram around the thing you want desecrated (and then see if your GM lets this emulate a desecrate spell). Make sure you cast lots of summon monster [good] spells and force the celestial beings to join you in your murderous ritual, so that you keep your alignment up. This is only recommended if your real alignment is Lawful Evil (If your response is "What do you mean, 'real' alignment, I'm as good as the rest of you", this is you).
7) Limited Wish and a lot of other spell duplicating magic can do this, but no spell-duplicating spell capable of doing this is less than 7th level (Shadow Evocation, notably, only duplicates Sorcerer/Wizard spells).
The 2nd-level Clr spell desecrate [evoc] (PH 218) does one of two things:
Either way, remember Chaotic, Evil, Good, and Lawful Spells says that
A cleric can’t cast spells of an alignment opposed to his own or his deity’s (if he has one). Spells associated with particular alignments are indicated by the chaos, evil, good, and law descriptors in their spell descriptions. (PH 33)
Meaning that a CN, LN, N, or eviler cleric of a similarly-aligned deity can pop off a desecrate spell whenever.1,2 Strangely, doing so—according to the core rules, anyway, although the DM may rule otherwise—is not an explicitly evil act, like channeling negative energy (PH 160) or trying to rook a mark into buying a cursed magic item (DMG 277).
The desecrate spell's effects on animated undead can be semi-duplicated by the deadwalker's ring (Complete Mage 126) (4,000; 0 lbs.), the magical terrain ebony ice (Frostburn 15), the supernatural danger dead calm (Stormwrack 13), the 3rd-level Clr spell defile ice and snow [necro] (Frostburn 92), and the 6th-level Sor/Wiz spell blackwater taint [necro] (Stormwrack 114). The latter two are also spells with the descriptor evil, but their existence increases your options: if a creature's unwilling to cast desecrate on your behalf, maybe it's willing to cast one of those spells instead? (Hey, it could happen.)
It seems every book has something for the necromancer, even if it's not an effect like desecrate (e.g. Sandstorm's black sand (20) and the corresponding 3rd-level Clr spell black sand [necro] (Sa 111), also having the descriptor evil), therefore, undoubtedly, further options for getting the spell desecrate's benefits exist, buried in sourcebooks.
Easier than all of these, though, is to just wear the following fancy magic ring:
This effect requires using an effect like the spell desecrate instead of just having a pile of evil ice, bucket of evil water, or that fancy magic ring, above. Instead, get this even cheaper, not-so-fancy magic ring:
That's cheaper than a wand of desecrate (5,750 gp; 0 lbs.) or even an eternal wand of desecrate (Magic Item Compendium 159-60) (4,420 gp; 0 lbs.) created by a death master (Dragon Compendium Volume I 29-36), both of which would require convincing the DM a good cleric could employ anyway. The lesser ring above is, I think, by far the easiest way for a good cleric to cast desecrate, although, certainly, other options exist.
1 They won't care, but druids are likewise limited. Archivists (HH 82-4), on the other hand, aren't and can cast clerical spells possessing any descriptor. Claiming to be an archivist is as easy as buying a book.
2 Even looser restrictions exists in the Eberron campaign setting.
For a character to be Good they must actually act Good and do Good deeds regularly, because they choose to be a Good person, and not simply to "top up" their align-o-meter.
In D&D, alignments are Cosmic Absolutes, not simply moral judgements of the mortal races. Regardless of the how a given people act or think, there are Greater Powers out there in the metaverse which exemplify the actions of Good, Lawful, Chaotic, or Evil. And even then, the definitions of these alignments are beyond the nature of those beings; the definitions are stitched into the nature of the metaverse itself.
If this Cler/Malc commits Evil acts regularly, they are becoming Evil.
Committing an act of Good to "balance it out" will do no such thing.
Option 1: Committing an act purely to adjust their alignment is an inherently 'deceptive' act. This is Chaotic behavior. It will not make you any more Good than taking no action at all, and it will over time shift you towards Chaotic.
Option 2: Committing an act purely to adjust their alignment is an inherently 'false' act. This act, by the nature of not being truly invested in it, has no meaning. It will not adjust your alignment in any significant manner.
In either case, one can argue that "A Good act is inherently Good" which is not untrue. But the argument also applies that "The act was committed for reasons that are not Good" - eg, to cover up their true nature. At best, they might receive a miniscule bump towards Good, but weighed against willingly and freely committing Evil acts, the most this could do is slow their progress towards evil. They would need to commit multitudes of these Good acts to ever come close to balancing out, and if they are committing so many Good acts, this would change their core nature towards Good over time. Because alignment is an absolute thing in D&D, "you are how you act, and you act how you are."
Looking at it from the perspective of players, it feels like he is 'gaming' the alignment system, and brings up the inevitable question of "If X can do it, why can't I? I want to be a Lawful Good murderhobo whose Evil actions simply don't count too!"
But with all of that said, there's still an easy solution to preventing them from short-circuiting the adventure! (Ok, so i get it; this was a year ago, and this particular campaign is probably long past it. But as always, advice for people who may encounter it in the future.)
Another thing that is in D&D, especially 3.5e, is the near omnipresence of magic items. Even by 2nd level a typical party has probably picked up a trinket or two, and by the time they are approaching 10th level many players are more "blinged out" than a flashy rap star.
So why wouldn't someone trying to disguise their evil nature have some little trinket or spell of their own that causes a Detect Alignment spell to read falsely? Misrepresent Alignment is already a 3rd level Cleric spell (from Races of Eberron). Lasting 1h/level, it seems like it could be used as just another step in your morning routine - get dressed, brush your teeth, comb your hair, fudge your alignment, and now you're ready for a day on the town!
At 10th level, they could even craft an item that automatically does it for them. This could lead to the "Aha! This is how he's done it for all these years!" moment after the climactic battle. After such intense scrutiny (and probably a battery of Detect Alignment spells from every investigating mage) the little amulet finally breaks, falling off his neck for someone to find and recognize.
Side Points to consider:
Detect Alignment spells might be considered either taboo, or outright forbidden as part of the investigation.
It might be considered an intrusion of privacy to just snoop around blindly with Detect Alignment spells, and have social repercussions. "Oi! Don'chu wave yer Eyes Of Snoopin' at ME you nosy basterd! -purse smack-"
It might also be forbidden as part of a "Fair and Unbiased Investigation". This is highly dependent on how formalized the investigators are, of course, but perhaps a long string of "He's Evil, lock him up boys!" followed by clear and obvious proof that they had no connection at all to the crime resulted in legislation that prevents Investigators from using that ability as any sort of evidence.
This is likely related to...
Evil does not (always) mean cackling madmen out to conquer or destroy the world. Evil people also live in the city.
Maybe the bar bouncer who enjoys getting too rough when tossing rowdy patrons out. Maybe the dock worker who's always got some kind of deal running on the side for himself. Or the politician who does the same thing. The Lawful Evil priests of the local temple of the Death God who commit human sacrifice each month, but only of willingly chosen volunteers of their cult. Etc..
Neither Evil nor Chaotic people are inherently unable to live within a city. In fact of the two, Chaotic is more likely to have trouble with it due to the general disposition towards Law and Order within a well run city.
