As I mentioned in the Chainmail influences in Greyspace thread
, Ahmut died 300 years ago. Stratis died five years ago and cast down his spear, which hit Ahmut's corpse. This transformed Ahmut into an undead creature (again, about five years ago).
It appears that Ahmut's people (the Baklien) worshiped Stratis, although they may have worshiped other gods as well. Unless he just decided to hang out with them because he likes wars.
From Dragon #286:
"In 699, Ravilla was shocked to its core. A horde of horsemen appeared on its southeast border, and Stratis, the God of War, was spied among their ranks."
Note that the Baklien are probably related somehow to the Baklunish, so it's likely they worship or worshiped in the past at least the older members of the Baklunish pantheon (Istus, Geshtai, and Xan Yae, plus Pelor, Nerull, and Boccob under their Baklunish/Baklien names).
Also from Dragon #286:
"The Red Scythe is a cult that dates back several centuries. It began in the outlying provinces of Ravilla, far from the center of elven power. Although quickly proscribed by the authorities, the cult grew. To humans, halflings, and gnomes living under elven rule, the Red Scythe offered not just power, but the power of life and death.
"When word of Ahmut's return reached them, the leaders of the Red Scythe knew their hour had come at last. Here was a champion of death, ready to do Nerull's will on Oerth. All that remained was to contact Ahmut and put him under the cult's command.
"The Red Scythe leaders were in for a surprise. They found that Ahmut retained all of his cunning and force of will. One touch from the lord of undeath slew the most powerful priest of Nerull, and the rest of the Red Scythe quickly fell into line. The cult would now serve Ahmut, not command him."
So the Red Scythe cult originated within the empire of Ravilla, in its outlying provinces, among cultists of a variety of different races (humans, halflings, gnomes). The Baklien lived outside the empire of Ravilla, and the only god canonically associated with them is Stratis. The Red Scythe came to Stratis after his reanimation since they liked that he was an undead warlord with a horde of zombies under his command. The Baklien might have worshiped Nerull (among other gods) for all we know, but the Red Scythe cult (as distinct from Nerull's worship elsewhere) didn't originate among them, but among Ravillan dissidents instead.