Anathema chains:
This item is very powerful in low rank games, as it often happens that an enemy will get extremely fed due to your teams missplays, these games ussualy fall into the category of unwinable, but getting this item early will not only give you useful Cdr and hp, but a chance to get some shut down gold along with slowing the enemies lead.
This Item is extremely good against immobile champs, such as veigar or jhin.
Zhonyas hourglass:
This item might seem troll but it is very sort of hidden op, first it is relatively inexpensive making it a great second item, after a mythic, its gonna boost passive ult and e damage, as well as giving you the extremely powerful item active, allowing you to do dodge key abilities. It also has an interaction with your e shield, as you can zhonyas under your e and it will persist till the end of the stasis.
Thornmail:
This is generally the optimal antihealing item, as the full effect procs on cc making it very simple for volibear to proc it, chainsword might be a viable option but it sadly lacks resistances.
Abyssal mask:
This item is very situational, and quite strange, it acts as a team buff but also a buff on yourself, as volibear deals magic damage as well, because oyur melee and will often find yourself close to the enemy, if you have an ap damage dealer or duo partner you will be buffing them additionally to buffing yourself, sadly it is a flat mr reduction, but it has an area of effect unlike Void staff.