The crystal and the staff are two of Sans's main abilities. The crystal was given to Sans by Gaster with the sole purpose of overpowering the human's abilities to save, reset, and use their stats. The crystal is a normal crystal with some writing on it, surrounded by orbs, each capable of something different.
The first orb shows Sans his stats (although Sans is not really limited to stats anymore, he still uses them to scale himself). These stats can indicate how destructive Sans can be; for example, it can show that Sans's blasters are capable of destroying an entire building.
The second orb shows how many abilities Sans has gained and their upgrades, which Sans can obtain through kills, special kills, or quests.
The third orb shows how many kills and special kills Sans has achieved. (Sans doesn't gain EXP or LV, but each kill buffs each of his abilities and his strength.)
The next orb displays "Quests." These quests are tasks Sans must complete while on his genocide run, such as killing a number of monsters or someone specific. For example, by killing Toriel, Sans completed a quest that gave him the fire ability. These quests can also be upgrade quests that enhance abilities Sans already has.
The last orb simply provides Sans with information about the enemy.
Now for the second item Sans obtained: the staff. The staff is a weapon Sans can use whenever he wishes, without limitation. The staff can summon or create energy to attack an enemy. It can heal others and create a shield around Sans, but it can also steal the life force of others and convert it into energy for the staff. The staff is now the only way for Sans to use KR (Karmic Retribution), but it has been upgraded. Sans can use the staff's KR to debuff someone by slowing them down or poisoning items. This staff can reduce enemies to a weaker state and can also be used for combat. For now, Sans has placed the crystal on top of the staff.
