Yes, a Bag of Holding could potentially destroy the Sphere
If we look at page 225 of the 5e SRD, under the magic item Handy Haversack, we see the following:
Placing the haversack inside an extradimensional space created by a bag of holding, portable hole, or similar item instantly destroys both items [...]
(emphasis mine)
From this, we can determine that the inside of the Bag of Holding indeed contains "an extradimensional space, such as that within a portable hole." The logic being that if the space in the bag is close enough to that of a Portable Hole to destroy the Haversack, it is similar enough for the Sphere of Annihilation. So, if the sphere is placed into the Bag of Holding, a random result is determined by the DM, which can include the destruction of the sphere.
Yes, there is no insta kill component unless whatever is touching the sphere is smaller than 2 feet in diameter
The Sphere of Annihilation is a 2-foot diameter sphere and states:
Anything else [except Artifacts, which are in the general case immune,] that touches the Sphere but isn't wholly engulfed and obliterated by it takes 4d10 force damage.
In regards to moving the sphere it states:
A creature whose space the Sphere enters must succeed on a DC 13 Dexterity saving throw or be touched by it, taking 4d10 force damage.
Therefore the answer is: yes, there is a saving throw allowing creatures to escape the relatively small sphere and everything that is not wholly engulfed in the sphere only takes 4d10 force damage. Most creatures will therefore be allowed to make a saving throw when you force the sphere into their space. In the case that whatever the sphere is touching is small enough it states:
The Sphere obliterates all matter it passes through and all matter that passes through it. Artifacts are the exception.
This means that you can obliterate items and creatures alike.
Tiny creatures
In general even Tiny creatures, the smallest category, have a space of 2,5 feet * 2,5 feet, although they are normally not that big, it's just the space they occupy. This might make you think that you should be able to obliterate Tiny creatures. But that is wrong as the Sphere of Annihilation states (emphasis mine):
A creature whose space the Sphere enters must succeed on a DC 13 Dexterity saving throw or be touched by it, taking 4d10 force damage.
This means that any creature that is not prevented from making Dexterity Saving Throws can always try to dodge the sphere and not take any damage at all. Possible conditions to constrain a creature so that it cannot make a Dexterity Saving Throws are:
- Restrained
- Stunned
- Unconcious
- Paralyzed
- Petrified
When you constrain a creature it would of course still take the 4d10 damage until it's dead and not be annihilated entirely. But you might be able to talk with your DM about the case where the constrained creature is entirely engulfed in the Sphere of Annihilation. It would be homebrew, but there is a chance to rule for instant obliteration if the creature is small enough. In the case that you can constrain the creature the 2nd level spell Enlarge/Reduce might be interesting for you, too. Under Reduce it states:
The target's size is halved in all dimensions, and its weight is reduced to one-eighth of normal. This reduction decreases its size by one category - from Medium to Small, for example.
This might help you obliterate enemies with your Sphere of Annihilation if you are bent on using it on creatures. I would recommend talking with your DM about methods to use the Sphere of Annihilation so that you are on the same page regarding occupied space and a creature's size.
According to RAW there is no way for an insta kill with the Sphere of Annihilation for any creature, no matter what size and in what condition the creature is.
Best Answer
Artifact is a rarity type in D&D 5e. As you can see in this filtered list from D&D Beyond, artifacts are the next level of rarity up from Legendary.
There isn't a specific guide to what makes an artifact an artifact, but by scanning the list, it becomes clear that these are one-of-a-kind items, frequently associated with iconic and powerful characters from various settings and adventures. Meanwhile, the list of legendary items contains various items that are incredibly powerful, yes, but most of them are items that there could feasibly be more than one of in existence.
Artifacts are incredibly powerful when equipped and attuned by players, which means that it'd be a tragedy if someone accidentally destroyed one with the Sphere, but more importantly, many of them are big evil macguffins. Allowing the Sphere to destroy them would cut many a quest quickly short!
For example, the Sword of Kas is an artifact that can only be destroyed by someone who possesses both the Eye and Hand of Vecna; this is a two-part artifact that imposes serious alignment issues on the attuned party. Allowing a single legendary item to destroy the sword would be anticlimactic to say the least.