The 6th-level Sor/Wiz spell howling chain [evoc] (Spell Compendium 116–7) lacks errata and omits many details that make it immediately playable. The DM must make several rulings on the spell to clarify its details. Below is how this DM would rule.
"When the howling chain spell effect comes into being, does it attack all opponents in reach or only attack one?"
The description of howling chain spell simply says, "You create a chain of force, which lashes out and attacks your enemies." It isn't clear exactly how many foes it attacks when it's created, but the spell's description continues, saying, "The chain attacks by attempting to trip your foes" (n.b. plural). The effect isn't, for example, given a base attack bonus nor is any precise limit placed on the number of foes.
Based on that, this DM would rule that when the spell's cast, the caster picks a crosshairs for the spell's effect to occur. Any and all foes within 15 ft. of that crosshairs are attacked by the howling chain spell's initial effect. (Also—just to be clear—, whenever a foe that possesses spell resistance is first attacked by the howling chain spell effect—for whatever reason—, the caster makes a caster level check to overcome that foe's spell resistance; failure means the foe is unaffected by the spell for the spell's duration.)
"After the howling chain spell's effect makes its initial attacks, does the spell effect only make attacks of opportunity?"
The DM must make a difficult decision: the howling chain spell effect either at the beginning of the caster's turn each round until its duration elapses attacks all foes within the spell effect's reach or does nothing, only making attacks of opportunity (1 per round per 4 caster levels of the spell's caster). The text is decidedly unclear on this point.
However, the howling chain spell is a 6th-level Sor/Wiz spell—a level higher than scrying and teleport and two higher than Evard's black tentacles, for example—, and it requires a 500 gp focus to cast it (which means no wizard that's been captured or whatever will be able to cast it and it couldn't be cast were the spell spontaneously acquired in the dungeon or in a town littler than a small town). Similarly, compared to other 6th-level Sor/Wiz spells like antimagic field, contingency, and disintegrate, the former—that the howling chain spell effect continues attacking normally—to this DM seems rather tame… and this from a DM who has a marked distaste for seeing his monsters humiliated via tripping.
Further, if the chain spell effect could only make its real attacks when initially created, the poor effect would be at the mercy of any foe that the effect failed to trip during its initial onslaught, a weird result for a 6th-level spell seemingly exactly for giving a wizard a respite from his foes. (At this point in the campaign it shouldn't take more than one or two reasonably competent foes making a full attack against the spell effect to deplete its hp!)
Given all this, this DM would likely edge to the former, allowing the chain spell's effect every round to continue trying to trip—and, right after, to beat on—the foes it can plus making its attacks of opportunity. Nonetheless, this player certainly wouldn't fault a DM for deciding the latter more appropriate and ruling that the chain effect only makes attacks of opportunity. (Although in such a case, his PC would wonder what the heck the chain was waiting for and perhaps yell at it a little… not enough to make it angry or anything, though.)
"Does the howling chain spell effect make attacks of opportunity against foes attempting to stand up from prone?"
This DM would allow the caster to pick whether or not the chain spell effect makes a particular attack of opportunity. (Perhaps the chain spell effect mentally asks the caster yes or no before making an attack of opportunity so that the caster must make such decisions blind if he loses sight of the effect?) Initially, I considered having the chain spell effect always make attacks of opportunity against any foe that provoked, but that would rapidly and uselessly exhaust its attacks of opportunity. Seriously, were such attacks of opportunity supposed to be frittered away, why give it any?
So, in the case of a creature standing up from prone, this DM would recommend that a caster typically refrain from having the chain spell effect make an attack of opportunity: the chain spell effect's only attack mode is to trip (not until after it trips a foe can it beat the foe so as to deal damage). This means making an attack of opportunity to make a trip attempt against a foe that's standing up from prone is pointless: the foe's not yet standing when the trip attempt is made.
This lets the caster use the chain spell effect's attacks of opportunity against foes that, for example, move within the effect's reach so as to attack the effect in melee. (Pity those brave dudes.)
However, this does mean foes opting to remain prone are, essentially, freed from the spell's effect (although, perhaps, still shaken—see below): a prone foe can crawl away, for instance, or cast a spell, or attack the effect with its crossbow… all with impunity. While this sounds like a drawback, the upshot is that those foes likely aren't advancing very quickly toward the caster! This is good. And foes who don't identify the spell in place—requiring a successful Spellcraft skill check (DC 26)!—should be unaware of this loophole unless informed of it by another.
"Does the howling chain spell effect possess any special senses?"
The spell's description doesn't say the spell effect possesses any senses. There are at least two choices: the howling chain spell effect either possesses the standard senses the game assumes all creatures possess or is omniscient, automatically detecting all foes within its reach.
While this DM was willing to say It's a 6th-level Sor/Wiz spell so it should be awesome, even he has a limit: also a 6th-level Sor/Wiz spell is true seeing [div] (PH 296) with its 250-gp-per-casting ointment, and even that spell doesn't really allow the subject to see everything. Rather than omniscience then, this DM would treat the howling chain spell effect as having Wisdom as a nonability (hence a +0 modifier) and 0 ranks in skills.
This will make it possible to sneak past the chain invisibly, hide from it, and so on. This DM think it's okay for foes that are willing to expend or use resources like that to remain unnoticed by the spell effect. (Bear in mind that attacks of opportunity can't be made against foes with total concealment.)
A more generous DM could give the chain spell effect a flat bonus on its skills—I recommend the spell's caster's caster level—, or even rule that the chain spell effect's skill bonuses are as per the caster's and that it possesses senses equivalent to the caster's current own. No matter what, though, this is something on which the DM should rule in advance.
Note: This DM really likes having spells that are in any way controversial brought to his attention before the caster attempts to use them in play rather than being surprised by them. (He also likes the player to do some research on his or her spells first to learn if they are controversies associated with spells being considered.) Although rarely analyzed in detail, howling chain is totally such a controversial spell. I can imagine fist fights over how this spell should work. For instance, even the duration of the spell's shaken effect is unlisted, and nobody wants to be shaken forevermore. (This DM recommends all effects of this potentially escalating fear effect end when the spell's duration expires.)