Here are my versions of these three in case they inspire you. For Demi-Gods / Avatars in my setting, they are at least CR 20 - A bit tougher than a 20th level PC and generally more than a match one on one with any single PC.
I tend to not limit them to be just a giant with spells. I want them to embody the profile/domain they represent. To that end, they tend to influence the world around them, especially in terms of altering the minds of creatures within a radius and the environment.
To that end:
I have Bane the Tyrant in my setting. Bane ascended to godhood from a Blue Dragon so he appears as something like a Gargantuan Blue Dragonborn. He created the goblinoid races and is always attended by a huge number of soulless goblins - an army for him to dominate. In my setting the Gods granted free will to humanoids when they realized that free willed beings could worship them, and worship gave them power. Most Gods created new humanoid races that were all free willed - but Bane thought it would be easier to gain power if most of his creations were not free willed and were bound to his control - so most goblinoids do not have souls or free will. They live to serve Bane. While he has clerics, he has more Warlocks as binding them to him is easier with a contract than with faith. In combat he plays like a humanoid Ancient Blue Dragon with three auras. The first emboldens the gobinoids within three miles and allows him to command any soulless ones within that range ... which means fighting him usually means fighting an army at the same time. The second allows him to destroy a soul when a creature hits 0 hp (no coming back with free will). The third impacts the environment in a 20 mile radius - causing it to crumble, decay and die. He spends little time on the Prime Material Plane.
I have Bhaal, but he is a 'House God' worshipped by a small clan. As such, he is a bit weaker. I last used him 25 or so years ago. The backstory was similar to the FR lore - an adventurer (Murderhobo to the extreme) that began to kill powerful extraplanar beings to steal their essence. However, he does not have some of the trappings we know from the lore and BG series ... specifically, no slayer form. He stole enough power to ascend to Godhood, and then continued to steal more. He began to hunt established Gods and set his sights on killing everything to take the power. However, his rampage inspired a resistance and he was imprisoned in a dungeon by powerful Angels so that he could be prevented from killing and starved. I had him appear as a medium sized being that could be confused with a vampire. When free, he inspires a murderous rage in those in his presence. If you can see him you need to make two saving throws. The first you make once upon seeing him and if you fail you enter into a mindless rage that requires you to kill as expediently as possible - anyone or anything that lives. The second save, if failed, gives you a need to kill regardless of whether you are in a mindless rage. If you fail this save you take 10 damage at the end of your turn if you have not killed something (with at least 2 hp) since the end of your last turn. If you fail either of these saves, the condition persists for several minutes after you last see him. His stats were those of a maximum level adventurer with ability scores in the high 20s for relevant stats. He dual wielded Vorpal weapons and could teleport as a free action between blood pools (effectively, any place where sufficient hp damage was dealt). He was a master tactician (I played him to the best of my ability) and he TPKed the party he faced - tool two of their heads on the first round of combat in a 2E game.
Myrkul does not exist in my setting, per se, but there was a Death God similar in my eyes. She was the God stricken down by the Raven Queen when she ascended. Her name is lost via magics the Raven Queen used in that act - and thus she is known as Nameless. She appears as a Huge Undead Dwarf in black robes with a Sickle made from bone... pretty cliche. She has never been stated out and used, but my notes on her indicate a pretty straight forward Lich Profile with a fear aura and the knowledge of all that had died before she lost her profile to the Raven Queen. She is stuck in a Shadowfell / Ravenloft domain where she is forced to watch the Raven Queen use her power to (sort of) do her job. The Domain is located in the Shadowfell in a parallel location to the Ultra Powerful Temple of the Raven Queen in the Prime - in an arctic waste with no settlements to be found within tens of thousands of miles (my world is big). She was intended to be a MacGuffin the PCs would visit to gather ancient knowledge available nowhere else.
