Abilitites and Powers
In life he had the powers of the Frost Frost Fruit the Shimo Shimo no Mi. This was a paramecia class devil fruit that allowed the user to generate auras of supercold temepratures. Much like a thematic and functional counterpoint to the Atsu Atsu no Mi of Don Acchino. But unlike the Atsu Atsu no Mi which cannot manifest anything other than literal heat the Shimo Shimo no Mi could gathre water molecules in the air, land, and sea and generate ice. In Zero's earlier days he believed this made him invincible and so he heavily relied on the powers of his Shimo Shimo no Mi. After his defeat at the hands of the Vice Admiral he realized this was a folly and so for two years trained his Shimo Shimo no Mi so as not to rely so heavily upon it. He learned eventually how to master it perfectly. In doing so he could freely manipulate ice and water by manipulating the flow of cold. In addition he became a competent swordsman and lancer, frequently forging weapons from ice that were just as hard as steel.
Zero's training alongside the other four allowed him to unlock pwoers usually far out of reach to other Devil fruit users. This was X-Limit, a state that puched the user far beyond their own limits, powered heavily by their devil fruit and force of will. In this state their attacks and techniques were amplified anywhere between 10 and 100 times. (Later uses of this are Luffy's Gear techniques and Magellans Hell Poison). While in X-Limit Zero would often carve the leter "X" into his forhead with his ice then take on the form of a massivee skeletal ice monster, freezing everything around him and attacking with unbelievable speed thanks to his ice body. Zero after death mastered this technique and was able to longer and longer bouts int hsi state without the considerable negative side effects such as unconsciousness or coma. by the time he was a god his pwoers in this state were astronomical and he had abbondoned the bulkier form for a sleaker, faster one. Despite its increadible strength it is a dire situation technique and for some reason can only be activated when the user is heavily wounded or exhausted. Of course after mastering and, and being dead, Zero could use it whenever but did not rely on it as it still had very few drawbacks in power and mental strength.
After the Time Skip he displayed great levels of competency in using ice forged weaponry. He was fond of a Broadsword which while heavy to all other was light to him due to his cold manipulation. He could fight on par with high level swordsman with a seemingly unwieldly blade and he could reforge it at will if it was damaged or if he needed a different weapon. While he nevere really acquired an formal swordsmanship trraining his skill are still quite comparable to such fighters of today as Mihawk, Vista, and Shanks.
Later in life Zero developed Haki, it was first seen during his defeat of the three admirals after the Seven Year War. Zero never fully unlcoked the secrets of Haki during his life time but was confirmed to have Haoshoku and most likely the other two. When he was revived from eternal slumber as a reincarnation to fight Fafnir gRey during Fafnirs attack on the Meridian Archipelago he demonstrated Observation Haki and Conquerers Haki in dodging Fafnirs soldiers attacks and more than likely used Armanments Haki, though even then his sills were not signifiant enough to turn his armaments black. During his wanderings as a spirit by way of the Orb of OVersoul, the Orb of Krosia, and later the pillar of souls he was able to train in his mental aspects. He aquired telikinesis and telepathy from his training in observation haki, and he gained emotional inspirational techniques from his conquerors haki. He eventual leared how to augment his boy with ice and haki aswell making him quite hard to damage.
During hsi wanderings as a ghost he learned through mental training how to gain the different powers of ghosts and their ilk. These powers include things like invisibility, intangibility, flight and supernatural attacks like wails and cold touches. In this state the bases of his Shimo Shimo no Mi remain and he can create a deathly palor of ice much more reminiscent of Brook's Soul Solid attacks. Though once he learned all this he learned how to manipulate souls and spirirts. This way he could gain the powers of other fallen allies temporarily by communing with their spirit. He could also assault mortals by attacking their soul. This evolved nto shadow magic where he could manipulate gloom, shadows, etc. These technique he discovered during his lonely trip through the Shadowfell after the Krosian Civil War. while very reluctant to use them for his fear of their immorality they are potent and easily some of Zeros most frightening attacks.
After Zero slew the corrupted God Gihennomyst the god repented and in his last acts passed on his Gospark to Zero officially making the Progenitor of the Krosians a fledgling god. He experimented with his newfound powers but found most of them to be empoered versions of his normal ones. After a bit of training he could grow wings of light and ice and cat down judgment fromt eh sky in beams of holy light and blistering cold. He could forge weapons from light aswell as ice and could regenreate at a rapid pace. In addition he had the ability to heal others aswell or protect them with auras. While most of these abilities served oly to augment and upgrade his already massive arsenal of powers and attacks they were potent in their own right and usefull for removing powerful foes that were otherwise resistant to his other powers, such as high classed demons.
When Zeros soul crossed over in Faerun he experienced a different world entirely. He discovered however that his access to a Devil Fruit in life gave him Magical blood here in death. He experimented with this and learned he could cast magic. At first he was just limited to ice spells but eventually learend other elemental spells like fire, lightning, etc. He learend he could also focus his power into his sowrd and creat close range attacks. Through interest he began to tamper witht he source of all magic, the Weave. In doing so he learned the secrets of other magics, not just battle magic, but rituals and passive spells. He continued to learn the abilities of magic after his second return from eternal slumber. Herer he returned with magic beyond comparison. Magic that altered gravity, time, space, matter, reality, and the laws of physics. While easily the most potent of all magics it required a purely calm mind and lots of concentration, time, and preperation to cast much more powerful spells.