Well, possibly. The spell was created for use on a world where all life only goes back a few hundred (human) generations. It could be that, on a world with millions of generations of life, the spell would be too underpowered to kill everyone.
The spell only killed humans who were related by blood through half dragon type beings right? That means it can jump species, but on our planet we don’t have any cross species pollination so this wouldn’t matter.
Er, wait. What if it was case on a liger?
Hmm, which would be limiting: the total number killed, or the number of branches. Would a short maturity time, massively fertile, multipartner group be hit harder than a long maturity low birthrate monogamous group?
How do area effect rules work? Realistically, something like a fireball would lose energy as it inflicts damage, someone standing at the edge of the effect would get hurt less if there were fifty people crammed in between him/her and the epicenter then if there was only two.
That would be my take if I was in charge. It took an epic level spell to kill several hundred beings. It’s simply insufficient to wipe out billions of people, much less obliterate all life on a planet. I would probably limit it by capping beings killed rather than branches. After all, the primary “thrust” of the magic is killing beings (apparently instantly and without any chance of save). The fact that it has a global span is powerful but I’d rank the insta-no-save-death as the real siphon of the power.
I dunno - it looks like a pretty healthy party dynamic to me. Tarquin may be their respected - even beloved - leader, he’s still first among equals, and his comrades don’t feel the need to indulge his every whim, and they’re not afraid to call him out when he’s being a jackass.
Do you really think the OOTS would risk their lives in order to, say, teach Roy’s sister a lesson?
One thematically appropriate limitation would be to restrict it to secondary targets that have a connection to the primary target through a still-living (or perhaps undead) relative. The spell hits the target, spreads to immediate relatives, then to their immediate relatives…but stops if the next link in the chain is dead. So if you cast it on an orphan, it wouldn’t even jump to their siblings, but if you cast it on someone with a very long-lived extended family, it could wipe out a large number.
It should prevent the planetary sterilization scenario, and curb the massive depopulation cases substantially.
That would be one way to do it, but it doesn’t match what we saw in the comic: Girard was dead years before it was cast, but it still hit his descendants.
It sounds like a Dick Tracy villain actually. 
I believe we were speaking of hypothetical implementations of a Familicide spell in a different campaign world. (Well, that’s what I was talking about, anyway.)
Lots of good lines. “I think I’m just making Greenhilt look badass.”
I kept incorrectly reading Tarquin’s bidet line. I kept seeing that panel as:
It took me several double-takes until I realized that Tarquin wasn’t blasting him with an all-caps MORON but rather was just using the proper noun “Miron” to address the guy. Heh heh …
Anyway, so can the Order just hop on the now-riderless pteranodons and fly away? What are the rules for controlling the enemy’s mounts once their riders are dead?
It wouldn’t really have the effect of preventing revenge killings. Let’s say I want to kill John Smith and make sure I kill off everyone who’s closely related to John Smith - let’s say three degrees of separation. So I cast the spell and kill off a few hundred people. And it works - there’s nobody left alive who was closely related to John Smith and will want to avenge his death.
But one of the people I killed was Mary Brown, who was John Smith’s second cousin - she was just on the edge of the spell. So several people who were just outside of the spell and were closely related to Mary are still alive. And now they’re going to want to kill me because I killed Mary.
Quite frankly, this was an inherent flaw in the original spell. The more people you kill, the more enemies you create by the killings. Even if you somehow manage to kill off every relative, you still have a huge amount of people looking for vengeance. Imagine, for example, that Haley had been a Draketooth relative and had been killed by the spell. Roy, Elan, Durkon, and probably even Belkar would be looking for the person who killed her even though none of them is related to her.
My guess is Haerta used the spell for other reasons. She probably used it to terrorize people. Her enemies knew that challenging her would result not only in their own death but in the deaths of their entire family. Haerta wasn’t worried about people seeking revenge against her - she probably bragged about all the people she killed and figured it was good publicity.
Durkon or Vaarsuvius can probably place them under magical control.
V. Durk’s “all but” out of spells.
But the more people you kill, the harder it is to know who the original target was. When V killed the dragon’s son, the dragon knew that a mage that was in her home was the killer. When V used Familicide it could have been a rogue mage targeting that dragon, or it could have been a mage targeting Draketooth, or it could have been someone taking a shot at Tiamat herself.
Belkar’s a Ranger, and they’re already trained to accept a rider. He could probably do it with Wild Empathy.
Belkar’s a terrible ranger.
The Familicide spell would likely be limited by HD (Hit Dice) rather than number of individual entities. Every creature in D&D has a number of HD which are used to determine hit points and to serve as an approximate indication of how strong the creature is. For player races, the individual’s HD is equivalent to their character level, while monsters have HD corresponding to how strong they are. When a cleric turns undead, for example, she isn’t limited by how individual undead she can affect but rather by the total number of HD she can affect. Suppose a cleric can turn 7 HD total worth of undead. If the cleric is trying to turn a horde of 1 HD skeletons, she would be able to affect 7 of them. If it were a pack of 3 HD wights, on the other hand, she would only be able to affect two of them. Typically, these abilities will affect the lowest HD creatures first. So if our cleric tries to turn undead on a group of five skeletons and one wight, the skeletons would all be affected first and the remaining wight would be unaffected.
What I’m trying to get at is the Familicide spell is probably limited by the total number of HD it can kill. Something really high, like hundreds or thousands of HD or some other arbitrarily high value. The cascading death effect would likely stop either when the HD limit is reached or there are no more valid targets, whichever comes first. The implication would be that while it is an extremely deadly spell, it probably wouldn’t be enough to significantly depopulate a world. At least, not without horrifying levels of planning. Of course, this is all just conjecture and will almost certainly never have any bearing on the OotS story, but makes sense both narratively and within the context of the rules.
Does vampiric gaze work on animals?
Yes, because of his low Wisdom and skill placement. Wild Empathy is just a straight level+CHA roll - it is literally impossible for his Charisma to be low enough to outweigh the ~+12-15 he gets from just being a high level Ranger.
A vampire’s Dominate ability mimics a spell called Dominate Person.
Animals are not “persons” in D&D rules. Spells with names like Dominate Person work on creatures of the Humanoid type - humans, elves, dwarves, goblins and so on.
Animals are a different type. For those you’d need Dominate Animal (a Druid spell) or Dominate Monster, which is 9[sup]th[/sup] level and therefore no-one we know can cast it.
Well, we’ve seen him try it at least once, right after V Charmed a muskrat…